I have been looking for an engine for my MC18 for a while, as I totalled the last one. I have managed to find one, nut it is attached to an ex-race bike and I am curious to know what I have actually bought.

Frame number MC18/1010644
Engine number MC16E-1144365

Frame looks to be an MC21 as it has the gull arm, but this may have been put on after. Wheels are Magtek, but 18" rear rather than 17".

The engine appears a bit odd. It has a dry clutch, with hexagonal barrels stamped F and F3, with a PGM II running on premix. Carbs are TA21B. HRC expansion chambers. Could be R6J SP?

Could it be an MC21 with a retro-fitted MC18 R6J engine? Or an MC18 R6J SP with a gull arm retrofitted?

Any help gratefully received. The important questions though are, will the crank be rebuildable, and (as I have not run premix since my Gilera moped) what premix ratio should I use (is it 20:1 unleaded to castrol TTS?).