I've been away from the forum for a while and came across this topic. What a shame that there are such dishonest people!
The thing you must be careful about on the Clark -4 or any differential for that matter, is what damage did the missing tooth do. Steve, at Peterson's told me that the tooth will almost always go through the gears and cause the case to get cracked. Usually the problem is much greater than just a broken gear.
If it were mine, I would take it to Peterson's (removed from the chassis to save $$$) and have them take a good look at it. Have it rebuilt the right way by real experienced mechanics, then take good care of it. I know it will cost some cash, but it will increase the value too.
I went through this with my 1402 last winter. Someone tried to piece it together with "repaired" parts. I went as far as having to get a new case.
But today I have a very nice cat, that is very dependable.
Steve said that Peterson's have looked at some Imps to buy for resale, but have stayed away from some due to the risk of a problem in the rear end. After my experiences, I would not give top dollar for one unless it has been closely checked out or recently rebuilt. If that's not the case then you have to steal it to cover your risk.