Heya

I think i have the same MC-18 set up as you the MC-18 mark 1, it just has 2 black boxes and a pgm1 box,
I have no idea why you need a total of 3 black boxes, i think by the PGM-3 (never seen a pgm-2 yet) they had it down to one box.

You may need the MC-18 carbs as i've been told the MC-16 doesn't have a throttle position sensor,
and there's a plug on the MC-18 loom for one, Not sure if the carbs with be a straight swap or not?
Let me know if you try. alway curious to see what's compatible.

I think the MC-18 and the MC-21 have a different valve solenoid set up, i think the MC-18 has one, the MC-21 has 2,
someone correct me if i'm wrong.

Not sure if the switch gear or flywheel/ignition pick ups are compatible either?

cheers

Robert