Right, looking at
this thread I think you can work out the overall ratio, on a standard spur, by
ratio = (approx) 186 / pinion
For a 12 turn motor I'd suggest starting around 10.5 or 11.0, so that's an 18 or 17 tooth pinion. 23T is 8.1 : 1, which sounds far too tall (although tarmac gives you the best chance of getting away with it - in grass you'd definitely kill it).
Your brushes will almost certainly be purple / grey at the tips - throw them away and fit new ones, ideally getting the comm (which will also be purple) skimmed at the same time.