It likely has a Buda-design four-stroke-cycle diesel engine. The older ones have AMBAC injection pumps that can be a mess to fix. If a 1969, it will have a Stanadyne/Roosamaster pump. Engine parts are very expensive so verify it's actually OK. I don't buy the "injector bleed" theory. Mine sat for 4 years and fired right up.
Not what I'd call a "small" AC crawler at 12 tons with a 516 cubic inch engine. I'd call an HD3 "small."