It looks like a 1/4 or 3/8 thick adaptor plate would work to mate the 420G to the M119 block. A 1/4 inch thick adaptor would have to be steel and a 3/8 plate could be aluminum but would need a pocket machined into it for the fly wheel. Personally I prefer the thicker plate as there would be more thread engagement.

There are a couple of things that need to be addressed;

The MB ring gear is smaller than the BMW ring gear by a few teeth, so in order to use the starter mounting holes in the trans, a BMW ring gear will need to be used. I want to know how many teeth are on the BMW starter pinion. If it is the same as the MB pinion, then I can use the MB starter I have for a mock up, if not, then I will need to get a BMW starter.

The 420G does not complement the M119 crank position sensor location at all. I think the easiest thing to do would be to move it to the top of the trans and re-index the flywheel to compensate for the new location. I have no idea if there is room in the car for the sensor on the top of the trans though. Another alternative would be to see if a shorter pick up could be sourced that could fit inside the bell housing.