How to sererate motor and remove sheared off bolts hayward super pump

[Kiss4aFrog]

I have a spare motor and pump that I'm trying to tear down and overhaul. Tried an impact but it didn't help. When I tried to take off the four bolts holding the pump housing together all four sheared off. I still have the whole assembly together (motor, impeller housing and body) and I tried to pry off the impeller housing to split it but it's still stuck. I think it's from the remaining bolt bodies being bloated by rust and seized in the impeller housing section.
Any helpful ideas on how to get this thing apart ??
Is the pump plastic and if so can acid be used to eat away the metal/rust to try and get the parts free ??
If I get it apart any ideas on how to get the rest of the bolt out of the pump/filter basket body section ??

Looks like:

 

[acamato]

Did you try to drill out the old bolts?

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[Kiss4aFrog]

I didn't try drilling. With the thing stuck together there wasn't any way to get a drill in there. I ended up soaking them and then prying the parts apart. The remains of the bolts that were still sticking in the motor mounting plate and the impeller plate are what had it seized together. The remains of the bolts had corroded badly and bloated and basically welded themselves in place. I thought the body was metal like aluminum but it's some type of plastic so can't use a torch. It looks like there is a metal thread insert in the plastic and mine are just so badly corroded that I'm thinking it's long past trying to save.
I have a good bolt from the working pump removed so I can get four stainless steel bolts that hold the assembly together and four of the shorter ones that hold the pump on. I was hoping to use the spare pump for a solar heater project but the motors in as bad a shape as the pump for being rusted up so I'm going to toss it in a tote for now and maybe try to figure something out later. Likely end up just tossing it out. I think it would be easier to just buy a used one in rebuild-able shape that isn't damaged.