I have a mid to late 90s Rex Marine exhaust system and one of the flapper solenoids overheated and burnt up. Anyone know where I can find a suitable substitute? Rex Marine didn't know. Thanks

Surely there's a part number printed on the solenoid body.

If not you'll need to know-
Input voltage
travel
force

From there spec'ing a substitute should be easy enough but will probably require a little fab work to mount and connect.