It's a direct plug-in, just so long as you change the wire harness, a few components, ditch the MC28 clocks etc etc.

To be honest, the MC28 was (for some reason) made difficult and expensive to derestrict, whereas the MC21 actually wants to be full power, but the Jap government put their foot down with a firm hand and said NO!

I've been playing with a programmable ignition on my 300 and it beat all the other's I tested against. That includes PGMIII (spliced), PGMII (spliced), F3 PGMII & NF5 PGMII. This is the way forward. Fully adjustable everything.

Biggest power gain wasn't so much the ignition timing, even though I was able to optimise it for the 300 motor and gain 1 hp over the NF PGMII at peak and a bundle either side, but it was the RC valve timing that really made the difference. Get that right and the mid range power just comes stomping in.

My 300 harness is not even a shadow of it's previous life as an MC21 harness, but I'm trying to suss out the 'plug-n-play' for MC21, and also trying to figure how to get it to work on the 28. But that's a little tricky thanks (NOT!) to the fancy electronics of the 28 clocks.

All said and done, best plan (for easy mojo) is to buy a 21 or 18, and then print out a pretty picture of a 28 to put on your wall.