Original engine and what kind of go kart and what engine/drivetrain should I go with?

[bbullsj]

I am building this kart for my nephew. He's 5 and i'm hoping it will last him a few years before he's too big for it. It didn't come with an engine or clutch assembly. What kind of engine would be good to use on it? It would mostly be used on grass or short grass fields. I'm thinking 15-20mph would be a decent top speed and it should have some torque. I don't know much about the amount of teeth on each gear and gear ratios. Attached are a couple pictures of what i'm working with. I think it's Chinese made by the brand of the tires. I did look, and the axle gear has 28 teeth, not sure of the spacing.

What are some ideas for type and size of engine and clutch and chain?

 

[Joe-405]

If it were me I would just put the oem style engine back in it. Especially if your not very mechanically inclined or can weld really good.

Looks to me like it was a 110cc or 125cc engine that resembles a pit bike style engine. They are about $300 new and complete with everything on eBay or other buying sites. And some of them have reverse also.

 

[bbullsj]

engine

I would have no problem welding or customizing to make a different engine fit, I just don't know much about the gear ratios and what size sprockets or what type of clutch/TC to use.