Late model 650/750 starter motor on early model

After cross referencing parts I believe the late model starter motor will drop into the early cases. However the early model starter motor is rated at .5Kw and the late model is rated at .6Kw. I assume this means it will crank harder? any foreseeable problems?

thanks

As long as the efficiency of the starter motor is the same (I have no reason to assume otherwise - it will definitely be pretty close), then yes, it will crank harder. The only real difference is a 20% increase in current on the starter circuit. As long as the starter circuit can handle 50A (600W/12V) you should be fine. I think that means 7 gauge in your units.
As far as mechanical problems go, I think you're a lot better qualified to figure that out but the only thing I can think of would be the starter gear or the starter clutch. I can't see a 20% increase in power shearing the teeth off though, but you may get some slippage on the clutch if it's not the same as the new models.

Edit: you may want to check the specs for the starter relay. I imagine the extra current wouldn't be a problem but that's just a guess.

The starter clutch and idler gear are the same between late and early models so imagine that will be fine.

OK. I just checked the relay from the 1980 750H (relay: 21164005, starter motor: 211631190) and it is used in the 76 750 twin (starter motor: 211631020).