What HP Upgrades can be done on a mid 90's 502 MPI Engine?

Upgrades are really only limited by your budget. You won't see any real gains from any bolt on stuff.

cheapest big hp gain (per hp) would be a used supercharger of some sort. 2nd cheapeast hp gain (per hp) would be buy exhaust that allows MORE cam, pull heads, do head gaskets, valve springs, 3 angle valve job, rocker arms, pushrods, lifters and cam. That will cost you about 100$ per hp, not bad, Smitty