RE: HOW DO I MAKE BALSA FILLER HARDER?
You don't have to worry about heat an epoxy. Unless the muffler or cylinder head is in direct contact with it, epoxy is no problem.
Some epoxies are specially formulated for high temperature applications (Circuit boards are made with a high temp resin). Even "standard" epoxies are rated for temperatures higher than what a model engine puts out. It will burn before it melts.