Your right to assume the grinding is the mains. I have over a dozen 20s, 30s, and Regulars and they all vary in the amount of crankcase noise they create depending on the amount of wear and abuse they have suffered over the years. I've found many times the bearing in the back of the bell housing that supports the clutch shaft is extremely rough and noisey. This bearing is about $30.00 and is easy to replace by simply removing the bell housing.
If you need to replace the mains, the engine must come out and be dissambled. I've done this to several tractors and can assist with advise if needed. New mains will cost about $300.
Again, check the bearing in the rear of the bell housing first. Often, replacing it will quiet most of the noise. I've seen (and heard) some Farmalls running that I was afraid to stand beside because they sounded like they would come apart any second but they ran that way for years.
Hope this helps Fred.