You cannot currently buy 21mm wheelsets for either IRM or MM locos as far as I know, certainly not from the manufacturers as you can with Accurascale locos. You would need to either extend the axles with bushes or replace them entirely.
The IRM bogies are not a plug and play replacement for the Cravens. At least some modification is required. The original bogies can of course be cut in half and a spacer installed to widen them.
The IRM stock that is 21mm ready is trivial to regauge. Just use a wheel puller to ease the wheels out on the axles and get a back to back gauge to push them back in to.
I believe at least some, maybe all of the SSM kits can be made in 21mm. The Provincial Wagons RTR models that I have are not really regaugable as they are based on GB models. I don't know if the kits are but @leslie10646 can answer that definitively for sure