Ian:
As has been mentioned, the best candidate for an M-420 drive is the Atlas B30-7/B36-7. At 36'-8" the distance between truck centres is only 3" greater than the M-420, and the truck wheel base is only 4" short.
For many years the M-420 was unique to CN, BCR and P&W, but the model has now propagated to numerous shortlines in Canada and the United States, with the result that a longer list of roadnames and paint schemes is now applicable.
Kaslo Shops was going to do the M-420 and HR-412 in all their variants in HO and N Scale. They only got as far as a few of the HO ones.
A conversion to an M-420 poses numerous challenges. The deck sits high and the girders comprising the frame can be seen underneath. On many N Scale chassis such as the B30-7 or B36-7, these run right through the portion of the chassis within the fuel tank. Another problem is those copper contact strips extending down to the trucks, which would be very exposed on an M-420 and would interfere with the frame and fuel tanks.
All that being said, I would encourage you not to proceed too quickly with this project just yet . . . . .

Tim