There needs to be some slack in that top link flatbar. I'm guessing he was getting that slack by fastening it with a chain at the back. It's not always had that luxury. that's obvious by the places on that flatbar where the paint is gone. This is where it has bent previously cracking the paint.
I'm also a bit concerned about the height of the 3pt pins? Are you sure your tractor will lift it high enough. Looks like it's been cut and changed at some time. Maybe elevated so a "taller" tractor could use it??
I wouldn't do anything with the gearbox until I had it useable in regards to the hitch and top link.
Unless you plan to use this cutter extensively, I'd not worry about replacing the gearbox seal. I'd do just what he did and pump it full of grease.
Keep us posted on your progress!!!!