OK, to shift the vibes, first we need to determine wether it is a rotational ,from the crank/cly or rotational from the spur driven starting boss, (in a conventional engine it would be called the crankshaft).
Is the viabrations a long the engine or across? perhaps a balance wieght on one side of the prop shaft could help, one would need to adjust it to fined the "sweet spot" or a additional wieght on the start hex, has any body had a go at this?????????
A.A.Barry