Think if that was done it would probably need some re balancing.

If it were a symetrical weight (say machined from steel or bronze bar stock i.e. homogenous material), the overall balance would remain much the same ? Just the overall mass of the rotating parts would change ?

Many moons ago I made a balancing shaft to have the flywheel of Mrs B's R65LS balanced to see if that would cut down on annoying vibes at UK motorway speeds. Before balancing the flywheel, the balancer operator chacked the balance of my shaft, which was made of 2 parts machined from steel barstock shrunk fit together - the shaft checked out fine & didn't require balancing itself before starting on the flywheel. Sadly, in this case, while the flywheel & pressure plate assembly balance was improved over factory stock (which wasn't actually that bad - it ended up the same class as an electric motor rated to over 10,000 rpm) the annoying vibes persisted.

Thinking along the lines of a disc-shaped additional flywheel mounted to, say, the outside of the generator rotor (or a ring shrunk onto the OD of the rotor) or attached outboard of the primary drive gear. IIRC Rotax themselves did the extra "lump" outboard of the primary drive on the trials versions of the disc valve two stroke engines, where "plonkability" and controlled pick-up were desirable features. Again IIRC Rotax offered various different rotor weights to provide the optimum characteristics for the intended usage - motorcrossers want different response & power output to trials riders, but the Rotax disc valve 250 was fitted to both.

Nigel B.