Make sure you get it to seal where the gearbox oil pump feed is on the MC21 casing. There's a hole in the gasket face, bottom right of the clutch as you look at it fitted. Down where the clutch cable bracket mounts. Your engine doesn't run a gearbox oil pump, so there isn't a corresponding hole in the crankcase of your engine.
It shouldn't be a problem to get it to seal, just be aware that you may need to mess about with some liquid gasket. Early MC16/18 gaskets seem to only be available from aftermarket suppliers now.