Re: 4-stroke & jackshaft in a stock 50's straight-bar Panther frame???
If you are looking for bolt on, with no fab work really your only option is the SBP kit. For the same money you could get a 79 cc remove the shrouds and mount it vertical...look at sportsman flyers bikes those are the 6.5 HF it would look similar but smaller. Weld on a jackshaft or have one welded for you. I know you talked about keeping the frame original but it really isn't collectable without the original parts and it's only original once. You would end up saving yourself a couple hundred bucks even if you had the welding done for you.
Motorbicycling.com - The Ultimate Users Guide