Most any decent gunstocker can refinish & rechecker a Win21 stock set. They are simple configuration and the checkering patterns are also with the lpi at 20 and 22 generally.
Nothing special to disassemble/reassemble if that has to be done.
If you are looking for a restoration to the Winchester Factory look of the era the gun was made, then search a little more carefully to make sure the person(s) understand what you are demanding.,,a restoration
The final look of the finish, the color, ect have to be just right for a restoration or it's just another pretty face.,,another redone gun.
Galazans/CSMC in NewBritain CT mfg'rs the Mod 21 under lic from the orig mfg'r and they do a tremendous amt of work on originals as well.
They do a lot of restoration and upgrade work on them bot orig mfg and their CSMC mfg. They mfg new parts and do repairs, ect.
Turnbull will of course 'restore' a Win 21 as already posted. They may have a standard price for these or at least a starting price for a Field Grade to give you an idea of their prices.
We did some Win21's when I was there but many more vintage SxS's like PArker, LCS and Fox. But times change and so does the market.
Another I can recommend is Brian Dudley in Rochester NY
Brian M. Dudley | Gunstock Craftsman
Does a lot of restoration work and is heavily used by the Parker and Fox collector and vintage shooter groups to do their work both restoration refinishing and complete restocking.
Does his own checkering work too.
Lots of his work can be found on the Parker collectors site.
Another in NY is Classic Gunstocks in Oswego, NY
Classic Gunstocks, Llc - Online
They do just about every thing oin the restoration line for vintage firearms too.
They own the Miller Single Trigger operation now having purchased it from Turnbull a couple years back after TB bought it out from the orig family.
Ken Waite and Dan May run the place. I see Ken every once in a while at a local gun range and shoot and local shows. Nice fellow with a passion for the work.