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
