Optima gearing with modern motor

[VintageRacer]

Having recently bought some electronics with the intention of getting my Optima running again I have a question about gearing.

As far as I can determine the Optima has an internal ratio 3.03:1.
The instructions of my 17.5 LRP X12 recommends 7:1 overall ratio as a starting point.

I am using an after market tufnel and steel 'spur' with 39t.

By my maths this means I need to use either a 16t (under geared) or 17t (marginally over geared) pinion.

These are higher than I used to race with, usually using around 12-15t pinions with a 27 turn motor and I remember people using 10t with modified motors.

Can someone confirm I have calculated this correctly and reassure me I'm right before I a) buy a new pinion b) over heat my motor.

If this is right what would be the effect (other than reduced top speed) if I use my existing smaller pinions.

Cheers.