Oh my, with a buddy who neglects a machine like that, he should of just given it to yah!
Well, this will be a nice rebuild blog. Hope to see more action here as progress is made.
I would 2nd the Seafoam or B-12. Bet the rings are dried oil in the weep slots. In some machines that get water down the exhaust pipe to the pistons and rings, pouring in the cleaners would help loosen it all up. Then one must slooooowly by hand crank the engine. Sometimes splitting the machine helps to do this.
As for oil, these Japan tractors are built in the days of METRIC and to the JIS specifications. Thus, the 10W30 diesel or the SAE30 are the norm for these Japan machines. The American old Iron machines need the 15W40. Don't let these small diesels suffer with that thick oil.