You might be surprised... I bet the old barrel will still shoot just fine.
It will likely be a problem child though going forward, you'll be continually fighting the rust issue most likely.
I would absolutely get the 22" barrel if you go that route. Not only will it shoot just fine, if you get the 26" it will be so top heavy you will hate it. Good luck!

Oh, and if it was me... I'd drop the $300 on a like new MK85 in stainless rather than a barrel, and keep the current gun as a backup/loaner. Better rifle by a mile, and they can be found in excellent shape for $250-300. Food for thought