Welcome to the forum and the Yellow Iron Disease!
The old 6 above has had a Dupont overhaul. Just as to what can see is sprockets, appears to have had rings already, lower rollers look to be contacting pin bosses, grousers are done, are extremely tight likely as like to roll off the idlers.
To get much work time especially pushing out a pond, gonna need under carriage, is a $$$ concern that he wants it gone. A full UC for this and its likely obsolete so rings changes are a moot point is going to set you back $16-24,000 in parts and because is press on drive flanges going to have to pull trucks. That means a lot more tooling & man-hours that you will want to spend.
Suggestion, as have been on the cheap side of this fence, shop around, do not latch on the cheapest you can find as fast become money pits or yard art. Newer and more expense is gonna hurt, although parts more prevalent, serviceability better, ROPS essential if going to work in trees and not a home spun pipe cage.
Will rapidly approach costs similar to hiring it done if not careful. And be tired to the point cannot get done as fast.
Want to add a Hydro tractor as a Deere or a Pedal steer slightly newer will do a novice better than a fully hand operated as these older beasts are.
