Money used towards an engine.

I made a go-kart with my dad back in 2014 I believe. We had an engine and it blew due to heat. The cart has been sitting in the shop using up space for over a year now, and we have been searching for an engine for quite some time! We still cannot find anything that will work. The go kart could be used as a tool or for fun, because it is large with full suspention like a dune buggy. We would need an engine with gears, probably around 300-700cc. It would be a huge blessing and would help out a lot. Thanks!