One small correction. Toni Clarks G38 drive uses a 32x18 at around 3000rpm.
All the manufactures of reduction drives I know off uses vee-belts, and not toothed/timing belts. The reason, as far as I know is, that the pulsating power delivery of a single cylinder engine destroys the belt rather quickly. Even my Mick Reeves G62 drive wears the belt quite a lot due to slipping at idle.

Mick Reeves / G62. Uses a wide "multi-vee belt" http://www.mickreevesmodels.co.uk/~m...ccessories.htm
Scale Warbird Prop Drives / DA50. Uses two vee-belts. Tried timing belts at first, but they were destroyed too fast. http://www.warbirdpropdrives.com/30601.html
Wolfgang Schlundt / G62 and G45. Uses four 5mm Gates vee-belts. http://www.modellbau-schlundt.de/
Toni Clark / G38. Two Gates 5mm vee-belts. http://www.toni-clark.com