I already have a spare drive shaft, universal w/ boot, pinion cup, dampers, throttle cables, kickstand bolt/spring. I think and alternator would round out my collection quite nicely.
Water pump, plus spare "O" ring cover gaskets.
Kickstand safety switch.
At least one carb float and vacuum piston. A set of float valves and bowl gaskets.
Thermostat plus "O" ring.
Radiator fan thermoswitch.
Clutch damper plate.
Starter pinion gear, rear cover gasket and anything else needed to repair a hydrolock.
Timing belt tensioners.
These are just a few more I'd have on-hand. My several-year project is to budget a certain amount per month and buy spares of things that need immediate or frequent replacement, and of those which may fail at some point (lesser quantities). I intend to ride these bikes until I can't ride and I'm flatly not interested in any other cruiser with the exception of a Rocket III.