Rick, the list rules are pretty specific about "no 'what's it worth'" posts. From a practical perspective, a 30-odd year old tractor with a runout engine is "worth" whatever someone is willing to pay for it. You say that you think you can get it for $1000 more-or-less.
As you know, a 420 is a very capable machine, if brought up to good condition. You say it has a 60" mower deck and a 54" blade. Any additional stuff, like a rear PTO or a 3-point hitch? What is the condition of the mower deck and the blade? Is it leaking hydraulic fluid, and does the transaxle drive the tractor in both directions?
I'd address the question of dealing with a runout engine by planning to rebuild it. That assumes that the engine starts and runs on two cylinders. And are you prepared to put 100-200 hours of "sweat equity" into doing the needed work? I am not one to change out an Onan engine for a repower conversion, which, I think, is the reverse of what a lot of people on this forum would tell you.
Also, do you have a "mission" for a 420 in addition to the tractors you have? Or are you just thinking about a showpiece in a collection. I have four garden tractors on my 2.5 acre place, and all get used. If somebody showed me a good 420 Wyoming (relatively) rust-free core with a mower and a blade to fix up and use for $1K, they'd have the $1K and I'd have the tractor.