As you know four stroke mowers generally don't have big counter weights on the crank like a Victa full crank so I would try a bigger diameter weight as the small 1.25 kg weight isn't very far from the centre line of the crank ,you could try a 2.25 kg weight .
You may need a weight in the diameter size of the flywheel or even a bigger diameter but 1/4 inch thick if you can fit this in under the base otherwise you'd need a heavy cast iron flywheel.
