personally i reckon an os18 in there may be too much for it, a 21 would be silly and undriveable - imo.

16/18 is as high as u can go using cen pinions and 38/42 for cen spurs, plastic ones will be fine, lighter too. dunno what the belts and pulleys will make of it tho - its got a lot more torque than the cen 16 or 18 motor, so be prepared for probs if u do it...