Go to an all battery store like
Batteries Plus battery store - All your battery needs for Laptop, camera, cell phone, camcorder, SLA, batteries recycling and much more and they should be able to help you on choosing a small one but with enough cranking power.
Love the headlight too. Small stuff like this looks pretty easy to make fit but I know it can get frustrating some time. You got a big project ahead but it seems like you're knocking it out pretty good. Keep us posted on how she's coming along.
I'll try to post a pic of the Spirit 750. Got her on a trade just the way she is.
Also go to
KZrider.com - Home and there are alot more folks with these types of bikes.