I'm struggling with the same thing, can't find a 21" Remchoke 20 gauge barrel. I've gotten several recommendations for Rose Action Sports in Pembroke, KY for a cut and rethread.
I spoke with Mr. Rose about it and emailed him a photo with the dimensions of the barrel. Based on that he's pretty sure it has enough meat to cut/thread for Remchokes. I'm not excited about the possibility of Tru Chokes.