Im in the middle of putting a '28 motor into a '21 chassis, and have a similar story, however it sounds like it is a bit easier for me in that I was able to swap over the the bits that werent long enough for the original ones that were. I now have a '28 motor running a '21 flywheel, stator and ignition pickups. Time consuming none the less.

Sounds to me like either, a) you get your hands on the MC21 bits with correct length leads and plugs to go on your MC21 engine or, b) you get your wire cutters and soldering iron out and lengthen leads and make your own plugs yourself.

Not sure if the colour coding on the wires follows through between the 2 models you are dealing with, but they do between the 21 and 28.