[Kevan]

In an ideal world you'd test a load of different motors and have a favourite that suits your driving style then simply stick to motors that are similar in performance.

As far as gearing there comes a point where driveability is more important than outright acceleration.  Don't forget if you go for large gear ratios i.e. over 3.5:1 I find you have way too much brakes and can't dial them out as I prefer a car that can still flow under braking without stopping on a sixpence...so that's how I find a preferred gear ratio by the amount of control it allows me to have on brakes.  

If the car is too slow with the gear ratio and brakes that feels best then it's time for a quicker motor...suck it and see 

We don't rally on magnetic track and high mag effect motors do have a big impact on a cars handling on magnetic track...good and bad.