Ok then, what parts are compatible with an MC21? I have read that cylinders can be used, with some appropriate modification to the cases.

If someone is prepared to modify cases, what other RS bits could be used, or combinations of parts be used? I've read that the crank tapers are different, but could RS pickups be used with a lightened NSR flywheel?

For example, could the standard 21 kickstart mechanism be removed & plated over allowing for the RS clutch side cover etc?

How close could you make the 21 bottom end like an NF5?