I have a live fire all metal PR. I did what, I believe, the original makers did, flat sand the sides and use a small file to file in the grooves. it looks the part 100%.
For whats its worth, Monsters in Motion (circa 2000) had a PR kit that used an actual Thompson M1A to make resin parts. So basically you got a resin perfect copy of a real Thompson. I used it to make my first PR, then upgraded parts over years until......I have 4 of them. I dont know all of your laws, but can you get any real parts?