My best "guess" is about 900-1000 rpm, figuring 3600 rpm @ the engine, ~1:1 at the belt, 2:1 at the gear box, and ~2:1 at the chain......all rough estimates, but not likely too far off.
Revised (I counted the teeth in a pic of a opened gear box and got 15x30, turning output into 1/2 input)...belt is close to 1:1 and the sprockets are somewhere between 2:1 and 3:2.....