Can't get a 21 pinion on 75 spur

[xray011]

I put a 6.5 motor in but the lowest pinion i can put in is a 23t.with a 75 spur.It has way to much top end to race indoors.was looking to put a 21,22 in.
so is there a upgrade motor plate or do i drill it out,or put a 81 spur on?
if i put a 81 spur on witch size pinion do i use?