Flywheel / ring gear holding tool
Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models
See Moto Guzzi Workshop Manual, MG# 12911801
Ray Hale originally suggested using a flywheel lock / flywheel holder for a Volkswagen Beetle. Not as good as the original tool (as it only holds the flywheel in place when the ring gear is fitted), but it works well and is far cheaper. Toss the bolt and nut it comes with. Just slip the hole over one of the protruding studs in the bell housing and the teeth will engage with the ring gear.
These are commonly available from many locations, including: