Hi you can quite easily change your 640 which is really a 620 to a true 660cc with a crank and barrel and that way keep your electric start, the 660 bore and stroke equates to a smoother engine as well so less vibes and a lot more torque, the problem with the 660 SMR engine is the high compression that could cause problems with crap fuel in remote areas and the first two gearbox ratio's are higher than the std LC4 engine so you dont have the std crawler 1st gear to get you out of trouble in tight going.
I know this because i have a 660 factory motard that i raided the engine out of to put in an old 620 frame i could register with an adventure tank for a trip i decided to do at the last minute and the original 620 engine rebuild was'nt near ready.
Basically what you get with the 660 is more power but usfully more torque a load more torque.
Your other option is to install a 660 cam with the matching valve spring kit with an FCR carb this will give you additional ponies with good reliability at a more affordable price.
Hope this helps i know this post maybe old, ive just joined after seeing your post but couldn't help commenting, your on the right track the 660 is not a big bore kit for the LC4 motor the older LC4 rally bikes run a 690 or 710 kit that is still reliable it just comes down to cost as always, one point i found getting on and off your bike just to kick start it everytime because you stop to take in the view a pee a quick chat "where the hell are we" etc soon becomes a pain when your bike weighs a ton, stick with the button at least that weight is down low. cheers