The parts don't swap, particularly the armature. The shaft diameter is different. It sounds like you've either got a battery that's going south, a starter that needs rebuilding, or an engine in need of a tune-up. There's no excuse for hard starting if the fuel and ignition systems are functioning propoerly, hot or cold. The engine should start in a maximum of 5 to 10 turns of the crankshaft, maybe 2-3 seconds of cranking.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!