Did someone say tractor?
So A-C in the 70's merged or formed a partnership with Fiat for it's construction machinery, their ag division was unaffected by this and continued on as simply Allis Chalmers right up until the mid 80's, when the company was sold to the same company that owned Deutz-Fahr, they ditched the orange paint and re-named the tractors Deutz-Allis. Then in the early 1990's Allis was sold off and AGCO was formed (Allis Gleaner Corportaion) AGCO brought back the orange paint and sold AGCO Allis tractors up until 2001, when they ditched the Allis name and just called the models AGCO, the orange paint would disappear completely a few years later when AGCO stopped selling their AGCO tractors and focused on their other brands, which include Massey, Fendt, Challenger, and Valtra. As far as getting parts for the A-C farm equipment it shouldn't be super difficult, any AGCO dealer will be able to get them, regardless of what brand they sell. We have a AGCO White, another now defunct brand AGCO owns, we just buy parts from our Massey dealer as if we were ordering for any Massey we have.
As far as that little 5020, they are similar to the Massey 210, aside from the hood and paint obviosly. There was a couple companies bringing in these tractors in from Japan at the time. Simplicity had their version too, although I beilve A-C owned simplicity at that time. These little tractors are tough, fairly primitive, but tough. I have a buddy with a 2wd 205 Massey which would be a little smaller than your 5020, that thing looks like it's been through a war or two but they use it for cleaning out their cow barn to this day. I was looking at buying a 210-4 which is identical to yours but it had some issues I'm not sure if I want to deal with. Parts for these things are next to non-existant. The only chance you have basically as finding some that are being parted out, there is some stuff available but I still haven't decided if it's worth buying or not. The guy wants $2500 cad for it, and it has a loader on it. It's not going anywhere in a hurry so I'll just keep thinking about it I guess.
Hope you have good luck with your A-C, try not to break anything lol