So I was filling the tub to do a decontamination and drain. I had the side panel off and when the water level/pressure got high enough I noticed water leaking at the black plastic fitting that houses what I figure is the temp sensor and connects to the silver metal pipe of the heater. (black piece on the end of the metal pipe left of the blue sticker in the pic below)
I observed a black plastic ring that had seemed to have loosened,backed out/away from underneath the main black fitting. This black ring had a slit in it, so I attempted to take it off from the metal heater pipe. Upon closer inspection this slit was actually a brake in the plastic and probably not meant for a means of getting this piece on and off the metal pipe, but rather should have no slit at all and be placed on the metal pipe by being slipped over the end.
I drained what water I had filled and disconnected the other tubing(plastic and yellow-ish, below the metal one tube int the pic below ) that went to the other nipple on this fitting. Then I was able to slip the leaking fitting off the end of the metal pipe.
Inside I also found an O-ring that was damaged.(In the pic below you can also see the black plastic ring I have been referring to and other bits of plastic, likely from that ring that was found inside this fitting)
So I called the Spa store where I found the previous tub owners had originally purchased the tub. I emailed them pics of the parts. They knew the spa and its heater model. I asked them for cost for these two parts(O-ring and black ring). They contacted me back later and said they're not available and that the whole heater needs to be replaced. $380 for the part and $100 labor.
Well I could hardly believe what I was hearing, that this could not be fixed and that the whole heater would have to be replaced.
Here is the model number from the heater. (Pic has been rotated 180 clockwise for easier reading)
Model No:IHH-6000-NHCW
Watkins Part No: 0927501
Laing Pat No: 6759
So I searched a bit and quickly found a replacement heater for less than what the spa store quoted me.
Link for replacement heater for $300
My first intention is not to replace the heater for less but to find these two simple parts.
What resources should I look to for finding these two parts. O-ring and black pastic ring that fit this heater.
If I can't find the parts my next soluton would be to connect the metal pipe and black fitting through a home made solution. This made me wonder -at what pressures would a home made solution have to with stand? What is the roughly the max pressure that would be at this junction?
Any ideas/suggestions for a homemade connection? I want to keep the water flowing through the black fitting as I think it's housing the temp sensor.
Note- the outside diameter of the metal pipe is smaller than the inside of the black plastic fitting as one would assume, but it's not a real close fit as that black plastic ring that is broken filled the diameter size difference between the two pieces. Also the end of the metal pipe is not a straight cut. In the earlier pic you can see the little pattern in the top of the metal pipe that probably allowed the black plastic ring to clip/snap into place. On the underside of the end of the metal pipe there is small semi-circular shape cut into the end. I'm wondering if a homemade connection could be made to be more reliable than the original. Perhaps some tubing and screw down clamps, with the repair tubing going over the outside of the black fitting with temp sensor rather than within as the metal tube with black plastic ring and O-ring originally did. Concerns for such a set up?
Here's a slightly more zoomed out photo of the equipment area.
Thanks for all your help

I observed a black plastic ring that had seemed to have loosened,backed out/away from underneath the main black fitting. This black ring had a slit in it, so I attempted to take it off from the metal heater pipe. Upon closer inspection this slit was actually a brake in the plastic and probably not meant for a means of getting this piece on and off the metal pipe, but rather should have no slit at all and be placed on the metal pipe by being slipped over the end.
I drained what water I had filled and disconnected the other tubing(plastic and yellow-ish, below the metal one tube int the pic below ) that went to the other nipple on this fitting. Then I was able to slip the leaking fitting off the end of the metal pipe.

Inside I also found an O-ring that was damaged.(In the pic below you can also see the black plastic ring I have been referring to and other bits of plastic, likely from that ring that was found inside this fitting)

So I called the Spa store where I found the previous tub owners had originally purchased the tub. I emailed them pics of the parts. They knew the spa and its heater model. I asked them for cost for these two parts(O-ring and black ring). They contacted me back later and said they're not available and that the whole heater needs to be replaced. $380 for the part and $100 labor.
Well I could hardly believe what I was hearing, that this could not be fixed and that the whole heater would have to be replaced.
Here is the model number from the heater. (Pic has been rotated 180 clockwise for easier reading)
Model No:IHH-6000-NHCW
Watkins Part No: 0927501
Laing Pat No: 6759

So I searched a bit and quickly found a replacement heater for less than what the spa store quoted me.
Link for replacement heater for $300
My first intention is not to replace the heater for less but to find these two simple parts.
What resources should I look to for finding these two parts. O-ring and black pastic ring that fit this heater.
If I can't find the parts my next soluton would be to connect the metal pipe and black fitting through a home made solution. This made me wonder -at what pressures would a home made solution have to with stand? What is the roughly the max pressure that would be at this junction?
Any ideas/suggestions for a homemade connection? I want to keep the water flowing through the black fitting as I think it's housing the temp sensor.
Note- the outside diameter of the metal pipe is smaller than the inside of the black plastic fitting as one would assume, but it's not a real close fit as that black plastic ring that is broken filled the diameter size difference between the two pieces. Also the end of the metal pipe is not a straight cut. In the earlier pic you can see the little pattern in the top of the metal pipe that probably allowed the black plastic ring to clip/snap into place. On the underside of the end of the metal pipe there is small semi-circular shape cut into the end. I'm wondering if a homemade connection could be made to be more reliable than the original. Perhaps some tubing and screw down clamps, with the repair tubing going over the outside of the black fitting with temp sensor rather than within as the metal tube with black plastic ring and O-ring originally did. Concerns for such a set up?

Here's a slightly more zoomed out photo of the equipment area.

Thanks for all your help