The genuine Piaggio ones are big money new and it's hard to find good examples used. The good news is that BGM has produced one at 1900g and an elestart one at 2100g, but wonder what the quality is like? At 1/2 to 1/3 the cost of a new Piaggio item, probably worth a try.

https://www.scooter-center.com/en/flywheel-bgm-pro-touring-2.0-electronic-1900g-vespa-largeframe-pe/px200-without-elestart-cosa-without-elestart-lml-125/150-star/stella-2-stroke-fits-also-models-with-elestart-note-with-this-type-of-flywheel-electric-start-is-no-bgm8225rt?number=BGM8225RT