I think for sheer ease of use, I'd go with two 6v RC car battery packs and wire them in parallel for 12v. Or you could just get 10 cells (1.2 v each and then solder them together. They're really small and flat and have the cranking power you'd need.
I have a Honda Super Cub 100 that only has a 2.5Ah battery and a Metropolitan electric start with a 3Ah battery and I can assure you that the R/C pack would be enough to power everything.
http://www.dynamiterc.com/ProdInfo/D...YN1452-250.jpg