I ran a Moki 1.8 in my Platt P51 ( at sea level ) It certainly wasn't overpowered but then it did come in at 22lbs. As well, I found that the tune had to be spot on for the engine to fly the plane with authority. A little on the rich side and things weren't as happy.

For these reasons, I would have no hesitation in suggesting using the 2.1, especially since you will be flying at altitude ! And spend time running in the engine, perhaps in a hack, so that it is at it's best in the P51.

As for fit, I will include a photo to show that the 1.8 fits well inside the cowl with only the exhaust stubbs showing. I think the 2.1 is the same ? dimensions .

Cheers,
Merlin Man.