Doable but expensive. I had Mario at POHF convert a Polytech 5.56 fixed stock gun to a 5.45 Krink a few years ago. Tula kit and Bluejack barrel.
The gun was a bit of a hatchet job, with a cut rail. Mario fixed everything.
Since the entire receiver is the registered part, the trunion and sheet metal have to be saved for the conversion. Some of the rivet holes and the magwell have to be welded up for authenticity.
The barrel has to be machined for the trunion which is a bit smaller than the Bulgy or Russky trunion.
I had Mario simulate the "X" and "Y" stampings and the trigger bump.
Sorry the pic is crappy but photobucket doesn't work anymore and imgur editing has messed up the resolution.
Maybe I'll upload another one.
Edit new photo uploaded...