More motor and gearing advice please??

[Guccipiggy]

Hi Guys, was hoping someone might help me out with my conundrum. I'm racing my new cat for the first time tomorrow and the 7.5 turn motor I ordered to use in it hasn't arrived. I'm going to have to use either a4.5 turn novak or a 10.5 speed passion. Given that its an open mod class, which would be the better choice?? Is the 4.5 going to be too crazy for off road? The track is fairly tight with a loose dusty surface.

I'm also having some trouble tracking down some reliable gearing info. For the 4.5t it seems an 11.5-12 f.d.r. would do the trick (19t pinion with 83 spur). The only info I can find on gearing the 10.5 asks for a f.d.r. of 8 (which I think will be hard to achieve with the kit spur). Does this sound correct??

Cheers guys.