I've seen a few on here singing the praises of Ballistic batteries. They look to be light and powerful.
There's another company in the US called EarthX. They make similar Lithium based, light weight batteries with built in smart chargers and battery protection circuits. People over here are starting to put them on 4stroke enduro/motocross bikes. I'm interested in the technology but they're 3 times the cost of a standard Yuasa.