gizmo, reducing a driven sprocket from 12 to 10 teeth without changing the drive sprocket would increase the output speed by 20%. This is not an impossible amount for the cutter drive, at least if the cutter is running without contact but it does move the cutting speed away from the optimum. More importantly though, that is a double sprocket, and the other part of it must be driving something. Whatever it is will be driven 20% faster as well - and it looks to me as if it includes the drum, so your mower will be wanting to canter around the yard bouncing off obstacles while you struggle vainly to keep up. To avoid this you have to either run the engine slower (which has the advantage of reducing the cutter speed, but it will also reduce available engine power) or slip the clutch considerably more than usual.
Realistically, if your Imperial has a Victa 2 stroke engine it probably has far more power than a cylinder mower can use, so running it 20% slower than intended is probably more or less feasible. You'd have to try it for a while and see.
