mate the spur your running will need a fairly small pinion if it will fit and the motor mount lets the gears mesh. A 12 turn brushed motor you will need around a final drive of say around 7.0 and work down from there.... with a internal ratio of 2.25 and a 70 spur and a 32 pinion will give u a fdr of 4.2 I think. so that will give you some idea.
But lots of things will come into consideration as tire size and track size and wether its fast or technical and so on.
Make sure your timing on the endbell is set mild so no where near 30* as with a brushed motor you need to find the sweet spot other wise your will wear the comm and the brushes out way to quick and make the motor useless.
Good luck and I hope anyone else see's anything I missed will chime in and let u know.
Fordy