Can I use epoxy putty to patch metal?
I would think you would go to an auto parts store and get a filler like Bondo .

Bondo has a pretty high heat rating and is not flammable in your conditions.  You will sand the areas, use mesh for bigger holes and apply.  You can't put this stuff on paint - it won't stick well.  And I would make sure the bonding area is rust free too.  This should be about a 2 hour job.