First you state normal idle is 800 RPM - this sounds high for a tractor, the idle spec for my 8N is 400 RPM. I am not familar with Farmalls so perhaps 800 is typical - I don't know. As you indicated energy storage is proportional to the square of the RPM. Therefore, if you reduce the idle speed from 800 to 200 flywheel energy storage decreases by a factor of 16 (800/200)^2=16. So at some point the engine will stall on the compression stroke.
Adding metal to the outer rim of a flywheel is an effective way to increase the energy storage. This method was popular for some dirt bikes (motorcycles) in years past. A ring was machined and pressed onto the OD of the flywheel.
However, I don't believe flywheel effect is always the limiting factor for idle speed. As the idle speed is reduced air flow through the carburetor venturi slows and the ability to mix gasoline with the air deteriorates. Also, the inlet passages are sized for higher speeds at very low speed the gasoline which is now poorly mixed can fall out of the air stream. As the mixture quality decreases misfire/stalling results. It has been my observation that stalling typically occurrs due to misfire prior to stalling on the compression stroke.
A smaller carburetor can maintain mixture quality however you still have the oversize inlet passages. Changing to a gaseous fuel (propane) could be used to eliminate the liquid fuel fall out problem and the resulting mixture quality issue.
Hit and miss engines use the change in RPM principle to harvest more of the flywheel energy. The engine is sped up (hit) then allowed to coast to a lower RPM (miss) before firing again. This is similar to deep cycling a storage battery. However, I don't think this large variation in RPM is what you have in mind for your Farmall.
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