I'd be willing to bet that of all of the current BMW drivers (not even counting prospective BMW buyers), less than 1% know what the original M1 was. We are the fringe.
Just use the M1 moniker. Sure there was a pretty cool car in the 70's with that moniker, but so what? To call it anything else would just create a glaring inconsistency in their marketplace. 'M' means money these days. Specifically, BMW gets extra cash for naming it an M1 versus some other inconsistent and confusing name.
.. or we could call it 'Jesus'.
Just pointing out that those two letters, in combination, are not sacred.