I am current rebuilding a 057XXX engine to 950cc with a few other mods. This may take a while as I am budget limited and I want to do it right. When it is finished it will go in and the original will come out for restoring and backup, I must keep it since it is a matching number engine.
If you have the data plate for the engine you are restoring, you could put it on your frame, and then you'd still have a matching number bike (most US loopframes have no frame numbers), the world's only 1972 or 1973 with original disc brakes!