Let me give you the short answer......I wouldn't.
It's a homemade kart...and there's no way we can tell where the weak points may be....It looks pretty solid and well bit from what little we can see, but being as we can't see, and you don't have the experience to know, just stick a Predator on it for now, spend the energy and money refurbishing the running gear...and hope it holds up....
That's a pretty old Tecumseh, and, looks like the kart's been around awhile, too....