Before you tear it apart to try and fix the lift problem, try adding a little extra oil to the rear end. I had one for many years and this simple fix cured the trouble I was having with the lift. My problem was caused by a plugged-up inlet filter on the hydraulic pump. Changeing that filter is quite a job, which requires splitting the tractor. My lift became eratic, sometimes it would lift fine, other times it would not want to lift at all. I was amazed that adding just about one quart extra oil above the normal fill level made that lift perform flawlessly over the last several years I owned the tractor. That extra allows oil to get into the pump by bypassing the plugged inlet filter. It may not be a "proper" fix but it sure worked good for me. My tractor was also around a 1970 and had around 6000 hours on it when I got it and close to 9000 when I sold it. It was starting to loose compression and probably in need of a rebuild. I knew this because it was starting to like a little shot of ether for cold starts. All in all, it was a good machine that I was able to sell for several hundred more than I paid. The only thing it needed in them 3000 hours, other than normal oil/filter changes, was a resister in the dash that controlled the readout of the fuel and temperature gauges. If you start to see funny readings on these, before you waste money on a thermostat (as I did), go get one of them $30 resistors from your local NH dealer (they know all about the problem which you will definitely have at some point if you keep the tractor long enough). I really liked that little 3 cylinder diesel which seemed to have more guts than a same-age 3-cylinder gas 3000 that I had, and it would also do almost twice the work on a tank of fuel. As long as it has had good maintenenece, and well under 9000 hours, you should see many years of good service from that tractor. I must say however, unlike a few others here, I still like my "red-belly" 1951 8n, better than I ever liked that 2000 and I cant imagine ever wanting to get rid of it. It is still easier to find parts for and surprisngly good on gas. With gas being so much cheaper than diesel these days, It may be cheaper to do low-hp work with that 8n than a modern diesel CUT.
