My brother has built a lot of Vincent engined speedway sidecars. His comment about them is 'nearly every bolt on them is a special'. Phil Irving was involved in designing both Vincents and Velocettes. With both of them, you are in the hands of the dealers. Other British motorcycles have parts which are more interchangeable. In both Vincents and Velocettes, the designs are good, but they are a pain.
I have actually seen a Featherbed frame with a KTT Velocette motor and gearbox successfully installed. I think there were lots of spacers in the mounts.