I also plan on converting a X1-CRT. What pinion/spur combo would work best on a 9920 / 7XL?

Ah, the age old question....How many cells?

14 cells

I would guestamate around a 10 or 11 tooth.

On which spur? Can I use the stock jammin ctrdiff metal 62t spur?

Stocks a 46 aint it?

62 teeth

Wow...uh well if your gonna run that, I would say a 14 or 15.