fixing aquarium heater with epoxy

Personally I would not risk it. You can do yourself SERIOUS harm if you end up electrocuting yourself if the epoxy fails/doesn't completely seal the cracks. Not to mention frying your fish.

Heaters are cheap. Just get a new one.

you can't fix it. I'd get a new one.

It may already be toasted since water was inside it. It isn't worth either the effort or the risk to try to fix it.

I would not want to rishk using it on several points. One is that the epoxy may not hold. I'm not too worried on the electrical end of it as it is pretty sure to kick a breaker once both sides of the line meet water. But then that is also a reason mot to use it if the breaker feeds the rest of the tank like the filter. Don't want to come home and find the filter has been dead for several hours. Third reason is the way the bimetal plates tend to distort once they get really hot. Once they are bent from too much heat, they never work right again.

Hi
it is very risky
you must change it with a new one

replace it

and give better instructions to your fish sitter next time.