What temp is the epoxy good to? My guess is you underestimate the temp in the exhaust system. Unless you got something good to 1000-deg, it's probably not enough.

It also has to be fuel rated. Most epoxies aren't.

This company sells something that might last for awhile....
http://www.cotronics.com/vo/cotr/newprod.htm

Two years ago, I used 2300-deg 907GF (I think) to redo some custom sidepipe muffler packing in my vette sidepipes. AFAIK, it's still in place. (I used stainless screen to hold fiberglass in a sound chamber....with this 2300-deg stuff as the holding agent.)

OTOH, you could have an exhaust shop weld another bung in it's place....and be done with it.