Generally speaking, its just shy over 60hp if im correct, if its an NA engine, the most you can do is get close to 100hp with increasing the air/fuel combo. That may be from bigger injectors, porting the intake/exhaust, bigger valves etc but that is a bit too much money right there if done professionally by a machine shop, you can do it yourself but one wrong move and you'll be looking for a new head lol
It might be a dumb suggestion on my part but you can experiment with adding a small turbo with veeeery little boost, but be vary of the fact that diesel engines work under very high pressure and adding more might turn it into a big grenade but if done moderately it should be safe. It's just that i dont trust older diesel engines made before the 90s, mainly due to runaway diesel scenarios, you can look it up on youtube.