A [not too] toned down version of the whole bike could be built for the street and replicas could be built for racing, too.
Our 350 racer has what was built as an identical twin sister out there somewhere, although it was built about 8 years ago, so ours has moved on some since that build. That machine's last known whereabouts was Denmark, but it was for sale at the time, so it could be anywhere by now.
A 500 Bullet engine, suitably tuned and fitted in a shortened Manx frame would make for a cheaper alternative to a real Manx Norton, while being just as quick and enjoyable / competitive. A Manx Norton can never be as short as our Manx framed machine, so cannot enjoy the handling and steering benefits that brings
For the road, 40 BHP, or thereabouts should be attainable and good enough to give a very enjoyable and exciting machine made along the same lines