Axel Stuber sells a very nice flywheel suitable for a 31 key organ;
it would probably be overkill on the 20er. The inertia provided by
the 12-pound cast iron wheel makes a huge difference in the ease and
smoothness of cranking. Visit

https://drehorgel.org/barrelorgan/equipment/index.html 

The only downside is the shipping cost to the US, probably compounded
by a tariff now.

Roger Wiegand
Wayland, Massachusetts, USA