Strange Spark Problem and Part Compatability ??

I'll try to make this short and sweet. I've got intermittent spark in one of the 4 cylinders of an 81 90hsp Evinrude. Sometimes I have spark during cranking with a spark tester, but do not have spark under load. I have switched plug wires, coils, and powerpacks and the problem has stayed in the same cylinder. So I'm thinking maybe the stator or the flywheel, but buying new is going to cost me at least $300 each, and I'm not even sure it will fix the problem. The local boat shop has an 84 and 85 90hsp stator, but I dont know if they are compatible with my 81'. Does anyone have any ideas whether either the stator or flywheel could cause such a problem and how to determine which parts are interchangable? This is geting very frustrating. Thanks for your help.