I don't know what bearing spacing can possibly have to do with vibration..Fore and aft spacing or up and down spacing ? Care to enlighten us ? The outside diameter of the crank bearings is such that the bearings are held tightly in the case..They don't move..The crank may or may not be able to move sideways in the bearings, but that movement has no effect on vibration..No engine is made with loose bearings..Only US engines have roller bearings on the crank, and they are just slightly loose up and down....
I have not seen very many BME engines, but the ones I have seen have no grinding on the counterweights..Balance on a single cylinder engine is a compromise, and some are better than others..The vibration of an engine is determined by the percentage of the piston, rod, pin, bearing, and clips that is counterbalanced by the crank webs, somewhere around 80%....It will not change after the engine is run..A heavy engine will soak up more of the vibration than a light one..
It's impossible to balance a single cylinder engine, unless someone has found a way to change the laws of physics....