Just saw this thread - great find on the old Rupp frame. Had several mini-bikes in the 60's - what a blast. I once built a mini-bike with a MAC 101 (two stroke racing cart engine) and that thing would flat out scream - but oh the front end wobble with those little tires!!

By the way, the cracks are caused by water getting inside the frame and freezing (just like a pipe in your house!). I've fixed several of them and you need to know that you can blow right through the tubing if you're not real careful. Of course, my experience was using a Lincoln arc welder and i probably had it set a wee bit to high!

a 3-5 HP Briggs is perfect for these buggers. If kids are going to be primary riders, I'd stay closer to a three horse and keep the final drive a bit higher or they'll launch like a Nike-Zeus rocket. If adults ride a five is okay with a bit more gear through the jack-shaft as we tend to have the "bulk" required to keep the front end down...

Have fun and let us see the results!
Glenn