To run the older slipper setup you need the longer spur shaft.
The retrofit XX tranny is probably just as hard to find.
None of these have been made in years. I have done what you are talking about doing a couple years ago with my Jrx-Pro and I am telling you it is not worth the hassle. http://homepage.mac.com/patridge/PhotoAlbum45.html
eBay is pretty much the only source for parts. Just get a new or used XXX or b4 off ebay.
If you had all the parts I would say it is still a pretty good car. The steering is not as good as the new stuff. You can use b3 rear rims for wheels.