a 650SX is light compared to sj or sxr or 750 sx or sxi or sxi pro.
they are a completely flat turning boat at speed. don't lean it over or the pump will just suck air and the ski won't turn either.
They slide out very easily. which helps for freestyle fun.
Hoodriding is easy on the 650.
In other words, you don't want one for racing, but for a almost free beginning freestyle ski, it has the right qualities.
For engines, look for a 750 sx or 750 ss engine as they have the lowest port timing for better low end. kawi's are known for not having much low end and the later sxi / zxi engines are much worse (but they make more hp)
750 engine swaps are pretty easy, use the 650 electronics and flywheel. See groupk.com for the how too.
Dave at impros woudl set you up with a good swirl impeller...(a must for freestyle)