Tipsy Nipper

Is it possible to fit a very light(ened) flywheel and starter motor to the VW engine? This is fitted in a Slingsby glider without a flywheel and the engine is close to the fire bulkhead

If you are thinking of adding a starter to a VW engined Nipper then I suggest you investigate the weight and balance implications of the starter, flywheel, battery capable of turning over the VW (including it's mounting structure) and associated wiring. You may find the resulting weight and balance calculation illuminating at least and probably prohibitive.
From my work fitting a Leburg system I am clear that adding any weight to a Nipper bites into the pilot and fuel carrying ability and is not to be undertaken lightly.
You are most unlikely to improve a very light aircraft by adding weight.

Jim