I'm with Boog on this one and while I don't claim to know-all on this subject, I can report that several of my motorcycling pals have tried lithium batteries on bikes (both modern and classic bikes) with pretty poor results. I am sure there are lots of people for whom Li cells have worked great - but there are lots of others for whom they have not.
BTW - I fly electric RC aircraft so I have a good deal of experience with lithium batteries and know the ins-and-outs of them pretty thoroughly. Lithium cells do not like being too cold or too hot and they do not like erratic charging or being drawn down to too low an SoC (state of charge).
Basically, lithium cells are like Goldilocks - they like it juuuusssttt right. Modern cells are more stable, but they remain a bit of a
knife-edge technology - and they is why they are so expensive and it is also why production electric cars such as the Chevrolet Volt have battery heating
and cooling systems. In a modern electric car - the battery and its attendant hardware and software is worth
more than all of the other parts in the car
combined (and that also is why electric cars don't hold their value all that well - an old electric car with an old battery will cost many thousands to refurbish).
Anyhow, it seems to me that the potential weight saving of lithium cells is simply not worth the expense and hassle. Saving 5-10 lbs in a 450 lb motorcycle means nothing and if you can tell the difference in handling and braking, I'd be surprised and would ask why you aren't riding for a factory Grand Prix team.
Just leave the fuel tank a gallon down every time you gas-up or don't eat lunch a couple of times and you will save the same amount of weight.