I was riding my 500cc Triton in practice at Calder raceway. Don Emde was riding an XR750 which seemed to be quite quick. It destroyed the timing side main bearing, so did not get to race against the fast guys on the Sunday. When I was building the Seeley, I thought of that motor. The bearing problem was fixed in later Sportster motors, and there is a 5 speed gear cluster for them. The only problem is the standard ratios are stupid. There was a guy with an XR750 at Broadford a few years ago. It was beautiful. He said he had built it from parts he had bought off the web.
There would be a very good aspect of racing an XR750 - I like to feel the hairs growing on my chest.