RE: fpe 5.8 with the shakes...
Worst single cylinder shaker I ever tested was the A&M 5.8 with the A&M crank...Totally unusable...Next was a small Homelite, or the Big Roper...
All singles shake, some worse than others...All shake at low rpms, and get better at higher speeds where the impulses get closer together...If the FPE uses the Sachs 5.8 crank and cylinder you won't get it any smoother, run it up to about 5500 and see how it does...Soft mounts make it worse....
All engines have small variations in the weight of the parts, so if one gets put together with all the weights on the wrong end of the tolerances you could get a shaker..There is no way to balance a single, unless someone can re write the laws of physics...You can sometimes get it smoother in a certain rpm range, but total balance is impossible...
this is why some single cylinder motorcycle engines run a balance shaft, to try to counteract the unbalance..It works pretty well to a point.......