Anybody have experience with really old Diesels?

I'd leave it alone as the car will run forever as long as the filters are clean. Most likely the pump cannot be turned up much, if at all. You'll have to find a old mercedes guy to give you any clues to any other benefits you could get out of playin with it. That thing is a tank and can take a lifetime of driving and abuse, as long as you keep up with the filters. My dad drove one thru some of the jungles of Vietnam to get back to camp before curfew.