I suggest you contact Aaron at Diesel Repower in TN (Tel: 423 965 4817). He has a shop full of barely used small Kubota engines, and could tell you which ones might work in your B6100. They retire these diesel engines with very low hours in Japan, and he has a source for them. I bought a D850 and installed it to replace the D750 in my B7100. The engine blocks are the same, but you may have to swap components. I had to use the old oil pan, exhaust manifold, flywheel, and hydraulic pump, but the replacement engine had only about 300-400 hours, and it gave me a few more HP. Was also able to use the water pump to replace the old thermosiphon system with just some minor modifications to the magneto bracket, radiator plumbing, and left side engine grille. The replacement engine runs perfect, and has very good compression.
Big job, but well worth it to get some more life out of your tractor.