Well, I won't guarantee it, but I have done amazing things with two-part marine epoxy. My local hardware sells one called PC11--two little cans cost $12 or so. It will set up under water and stand up to filter pressure when cured. The only way it will work in your case is if you do a collar all the way around and smooth it out onto the thread and the filter wall. Otherwise the water pressure will break it free or at least leak.
Also,I used some stuff in a tube from lowe's in a purple package..It was a adhesive that stand's up to water..I burned up the shaft seal&plastic around..A pool guy told me about it..I cleaned out the housing&glued the seal in place...It worked for that..I know I will have to replace the housing next go around..But it was a quick fix for the year..
If that is a Pentair/PacFab FNS like I think it is, then you just need to undo the union connection, grab the raised "ribs" on the inside of the bulkhead and tighten using those. It looks like your bulkhead may actually be damaged too, and the oring should be replaced as well. I'd order the 190141; it is a replacement bulkhead and o-ring.