It's also possible that the tank has grown algae while it sat for 2 yrs. If you drain the tank, you'll want to unscrew the tank screen, remove it and clean or replace it ... reinstall with a new gasket, of course. It's possible that this screen is gunked up to the point where not enough fuel gets through to let the engine run smoothly. You can also treat the tank with Biobor (marine supply), which will kill the algae, though not particularly clean it out. I treat my tank every few months as a preventitive measure. I can't let the tank get to the last few gallons or it will cease to run, or run with very little power. I haven't changed my screen yet because it runs OK as long as I keep the level up (and I have other things that need doing more).

Very few owners know that the valves are supposed to be adjusted every 15,000 miles ... the POs may have just run it until it quit, and decided that the engine was shot. The result will poor combustion.

Diesel fuel doesn't go stale like gasoline --- if the fuel in your tank is clean, you can run it OK.

Diesel Giant has a nice DIY section, or choose Diesel Forum on this site and do a search -- there's a wealth of knowledge here (from much better mechanics than me).