Don't worry about getting all the rust out, just clean up what you can and button it back up so you can fill the crankcase up with Diesel fuel- - - -like up to the level of the oil pan rail. Then spin the engine with the starter Or take a SLOOOOOOW lap around the farm towing with the loader with the transmission in high gear. The spinning crankshaft will splash the solvent into places you'll never be able to reach with a brush.

Even if you end up rebuilding the engine, which is likely, a lot of preliminary cleaning will already be done for you.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!