Mike, it is pretty unlikely a serious mass-production engine from a major manufacturer will be out of balance, unless it is the one-in-a-hundred-thousand that had a defective part installed accidentally. Also, remember that the clutch only disengages the clutch shaft and sprocket: both sides of the whole clutch are bolted together, and rotate whenever the engine rotates.
I don't know if you are interested in this, but single cylinder engines, including motorcycle engines, have a reciprocating weight (the piston, and the upper part of the connecting rod) going up and down rather than round and round. This causes engine vibration. On large, low speed engines that don't have to be light, it is possible to use balancing systems such as the Briggs Synchrobalance (a largish weight in the sump moving out and down out-of-phase with the piston), or a balance shaft rotating in the opposite direction from the crankshaft. For most engines though, you just put up with the vibration. Side-by-side twin 4 strokes have exactly the same amount of unbalance as singles. Horizontally opposed 4 stroke twins do not have the problem - they have a pair of pistons exactly out of phase with each other. This is the reason the old Briggs horizontally opposed side valve twins were so well-regarded by ride-on owners.
There is something that engine manufacturers can do to reduce the single cylinder vibration. They can increase the size of the counterweights on the crankshaft by half the weight of the piston and upper part of the rod. This halves the vertical vibration, but adds a horizontal vibration of half the magnitude of the original vertical one. The technique is called "half balancing" for obvious reasons, and it is practised on all but the most rudimentary of single cylinder engines.
So, SB45s, and single cylinder walk-behind mowers of every type, have engine vibration. The decks have to be designed to be able to stand the resulting loads. However if the clutch on the output shaft is unbalanced as well, the loads go beyond those the mower manufacturer allowed for.
