It depends greatly on the type of epoxy as to what it's upper limit temp is. Some epoxies have very low upper temp points and others are very high. Some epoxies can handle low exhaust
temp for Muffler
repair etc. Most fully
cured epoxies don't melt, they char. Epoxies are thermosetting as John pointed out. In the very intitial stages of cure, the epoxy may be hard to our touch, but there is enough epoxy that has not cross linked and that when heated, it will soften and allow those chains to continue to form, with the end result of a much harder epoxy. That's why they go soft at first, but will get harder and much harder than a standard room temperature Cure. But fully cured epoxy mostly chars rather than melt.
However, if the motor is getting hot enough to melt or burn epoxy, then your windings are in great danger
of failing and any plastic on the pump would have melted. So all in all, Epoxy should withstand more heat than any plastic. The maximum temperature of a motor should be still cool enough to allow your hand to touch it. any hotter and you will melt the winding insulation
. There are special insulations used in special high temp motors, but I doubt a bilge
pump will be considered that special.