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