It looks like the Serpent S411 internal gear ratio is 2.0 based on a quick online search.

You will want an overall final drive ratio (FDR) of 4.2 to start with a 17.5 motor (this will change with a 21.5)

Basic formula:

FDR = (Spur / Pinion) * Internal Gear Ratio

Rearranged to find pinion:

Pinion = (Spur * Internal Gear Ratio) / FDR

Pinion = (84 * 2.0) / 4.2

Pinion = 40

So a 40 tooth pinion should be a good starting point.

I hope that helps,

Blake