'21 motor into an '18 is easy.

The '21 has a GPS which also acts as a neutral switch on the '21. You need to wire this into the '18 harness as you will otherwise have no neutral light.

You will need to either swap all your pick ups from the MC18 to the MC21 as yours is a R2J or make sure that the '21 motor comes complete with fly wheel and pick ups. They differ in size so can't be swaped between those two models.
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If I have to take the carbs off once more...