im worried about the epoxy. Are there any that can handle high engine temps? or exhaust temps (muffler tip temps)

If it's a regular polyester tool, I would post cure it very slowly up to 50 degrees. Don't just put it in an oven at 50 degrees, rather do something like 30 degrees for 2 hours, 35 degrees for two hours, 40 degrees for 3 hours, and then go 50 degrees for 16 hours. The slower you can ramp it up to 50, the less likely it is to distort and/or lose gloss. Once cured at 50 for 16 hours, you should be able to make your part in it and do a 16 hour cure at 50 degrees for an initial cure, then allow it to cool to room temperature before removing it from the mould and do another very slow post-cure up to 100 degrees. You'll want to test that the epoxy resin doesn't stick to the polyester gelcoat, I've never had any issues with that when using a semi-perm release agent bit that may just be the specific materials I'm using.