If your experiences are anything close to mine ('81 4420 443 head, bought with about 2200 hours), then believe it or not , those hours are enough to cause a good bit of wear if it has spent it's life doing corn.
Your chains on the head are probably about done for, check to see how much more tension can be adjusted, but I'd suspect if they haven't been replaced yet, it will be soon. Check for cracks or repairs on the chaffer frame, they like to shake themselves apart even if the bushings are good. Elevator chains and elevator sheet metal are getting worn.
Just take that manual, keep looking it over, stand there at the machine while you're reading it, if you don't understand, keep looking, touching feeling, reading, and moving it and you'll start to make sense of it. Lube it up good, run it slow while walking around it watching and listening (be careful, no leaning or touching now). After you've run it for a while, shut it down and feel it all over real good looking for anything that seems to be too warm. Lube it again after it's a little warm. Follow the adjustment recommendations in the book for starters, then tweak from there. You can always come back to this forum for adjustment pointers.
Should be a decent machine for you. I like mine and have learned a lot, and still learning!
Si Quaeris Peninsulam Amoenam Circumspice