Just off the top of my head, I'd say check with TPM Outfitters or IGF (Investment Grade Firearms). Both do outstanding work, there was a thread on here awhile back stating that IGF is very backed up and had long t/a times currently, it's a small 1 one operation. I've never personally used them but have personally seen guns that each have done and they're top notch.
I've used TSC (Tactical Supply Company) exclusively for my HK conversions and their work is outstanding but I think they only work on HK roller lock weapons, unless someone knows something different.
I'm sure others with more expertise will chime in shortly