How difficult would it be to replace an underfolder on an AB2 with a fixed stock? Worth the effort? thanks.

For what it would take (receiver and rear trunion swap) you would be better off selling the UF'er and buying a FS.

You don't have to swap receivers, you would have to weld up some existing holes and add some new ones.  A lot of work that requires some real skill.

Mako/Fab Defense offers stocks that are a direct replacement for the underfolder stock (no mods to the rifle).  Can't link right now, I can add it later.

Easier and cheaper to sell the underfolder and buy a fixed stock rifle.