I think the key to the longevity of the old gas engines lies in their rpm's. The gassers from the 50's ran about the same rpm's as the diesels of today, resulting in solid long running engines. I disagree with the comment that diesels are better for setting. The fuel is the only thing that sets better, diesels are more prone to stuck rings, resulting in poor/hard starting because of low compression. I have had several diesel tractors that required dragging for quite a bit (the heat produced from turning the engine will help the rings seal) before firing. Most injector pumps are also prone to leaking after setting for long periods. I have several gas tractors that set all winter and with a freshly charged battery, and a gallon of fresh gas fire right up with no problems. I even picked up a Farmall H that had not been started in 15 years, within an hour she drove onto the trailer.
As for parts I have a 41' Farmall H that I get a lot of parts for right at our local Motor Supply. The rest are easily obtained at a nearby Case/International/New Holland Dealer. I also have a 53' Ford NAA that is equally easy to find parts for. The 284 was made by komatsu and has a mazda engine, which would lead me to believe parts might be harder to find but I don't know that for sure. Any model I was considering I would contact a dealer and ask about parts availability. A lot of stuff is available on line but nothing beats a good mechanic and parts department when you get in a bind.