Kid's ice motorcycle conversion?

Hello, Long time lurker, first time poster. My kid has a mini dirtbike as seen here... http://www.bajamotorsportsonline.com/product_info.php?cPath=3&products_id=92

It ran pretty well, for one half summer. One day wouldn't start. Tore down the engine and he bent an exhaust valve. Long story short, his grandmother (purchaser) hasn't bought the replacement parts needed for me to rebuild it (as offered). So, I'm thinking of converting to electric. The bike could do 50-60mph on an ice. I don't need it (or want it) to go that fast. I'd say 30-35 is enough, but with plenty of torque. Can someone recommend a solid (hub?, mars?) kit that I can use with as little fabrication as possible. I do have experience with small electric scooters (controllers, brushed motors, sla batteries, hall throttles). Or can you tell me what to stay away from?
The motorcycle doesn't seem to have a lower support bar (to mount batteries), but I suppose I'll figure out the battery situation after I fit a motor/controller/throttle.

Thanks!

Adam