The R-series has a different crankshaft, so it will not be a straight fit... As the head is basically the same on E4 and R (R has the fuel pump on the head - this is the only difference), I think even the camshaft has the same lift and timing as the single carb Maxi. The differences boil down to
- shorter stroke than a 1750
- Carbs, inlet and exhaust
- ignition (electronic distributor)
There is not enough space to fit the carbs from any R-series Maestro to the Maxi, so the advantage of the well designed manifolds or double Webers sadly has to be skipped. (It has obviously been tried!) The distributor from an R-series Maestro can make a good addition and is known to have been used. I don't know if the SU or Weber spec suits the Maxi better though.
There are ways to extract between 110 and 120 reliable PS from the Maxi engine: Mostly through better manifolds - downton did such things, also BL sold special tuning kits to bolt on. You could also try to adapt the EFi system from a Rover 216 EFi to the E-series. This has been done, but a manifold needed to be made to fit the engine bay.
The Maxi's engine bay is much narrower than the 1800's, fitting an E6 will be a very massive task...
Regards,
Alexander
PS: The R-series MG never got that carb cooling fan. And has massive hot starting problems...
