Can you give us more history on the engine, like use, hours, even fuel used? For example, if the PO tried to use
an alternate fuel, the rings may be gummed up.
Can you adjust the valves? If so, check to see that they are in adjustment. You may not be getting full compression otherwise.
The higher end small diesels are pretty robust. Hard to imagine that the engine could be worn out on a garden tractor unless it
received exceptional abuse.
It came from a friend of my grandparents estate. I bought a Ford 1000 with a front end loader from them 12 or so years ago. His wife couldn't get it started then. We found there was water in the diesel, they had a 250 gallon tank the were using for a couple tractors. after getting some clean fuel in it we discovered two of the injectors had broken internals, guy at the local kubotta dealer said it looked like they may have had water in them that had frozen and caused the parts breakage. Three rebuilt injectors and it still wouldn't even try to start, even with either. At that point she didn't want to spend anymore money on it.
Long story made short, I replaced a hot water tank for her and she asked if I had any interest in the Bolens and if wanted it I could have it.
Thats where I'm at as of today. Valve adjustments all look good, glow plugs work, and its getting fuel... compression test shows 150psi on 2 cylinders and 170 on the third... said she drove it to where it was parked after mowing with it, that it did run rough and blew a good amount more black smoke than usual but that it was running when she parked it. hour meter shows just under 800 hours, I would think it should have lasted more than twice that many hours. I've got a John Deere thats got almost 4000 hours on the original engine. I'm guessing a severe lack of maintance as the oil came out black and tar'ish and the air filter was completly plugged. one curious thing was the back side of the air filter had a lot of soot in it, im not sure how that could happen.