I believe the 501 series is a "pittman" type mower. One of these would probably work fine under the apple tree, but it's my understanding that this type mower must cut pretty close to horizonal. If you need to cut on an angle I believe a "wobble-box" type (pittmanless) sicklebar would serve your purposes better.
Some models that I came up with to consider were
JD 350
NH 451, 450, 455
MF 238 (with cast head, not welded)
Ford 505
IH 1300
Case 10
Ford 515 (apparently has a bad reputation)
I may be wrong about some of the above being wobblebox type mowers, but tried to find some info before I made the effort to look for one seriously. I recently bought a NH 451 that looks to be in good condition but doesn't appear to be set up properly (routing/fastening of lift cables to cutterbar). This model is still manufactured (probably with several changes since mine was sold) and I have ordered an owners manual that should help clarify my situation! I believe parts and manual are available for the IH 1300, probably the JD 350, and perhaps others. That would be a big factor to me in deciding to buy! Good luck on finding one.