[Graham]

It would make sense to limit the smaller engine cars to slower motors e.g. 21k short motors
...the bigger engine cars to faster motors e.g. 22.5k short motors
...and modern touring to include long cans e.g. 23k (SlotIt DTM motor is 23k)

...for example

Actually the slot it DTM models are 21k rpm, but the basic idea is worth considering