In the real world of the Army and active service Devils Own, if the numbers were very feint and on Fazakerley rifles, feint to the point of invisible, we'd re-stamp the rifl serial number into the left side of the butt socket but leave the original markings intact.
If it is lightly bead blasted, phosphated and then painted then oven baked, then the original markings will still be visible and 'probably' readable but at least it'll be as original as it was ever in UKMilitary service. If yours is marked thus, you know its been there, done it, got the tee-shirt etc.
Oh, yes.... As for the mix and match parts, well don't for heavens sake let that worry you in the slightest. You are being fooled into thinking that Armourers would go through big crates of spare parts just to find the specially marked whatsits or thinggies to fit onto a BSA marked thingummyjig! Nothing could be further than the truth. Armourers used to be allocated 200 No4's, No5's or L1A1's or Savage Shotguns or Stens or whatevers. And guess what he'd do until the job was finished. Yep, he'd just plough on and on. I don't EVER remember looking for a special part. The troops just wanted tham back stripped, repaired, accuracy and range tested in the shortest time and that's JUST what we did.
Every job was done to the best of our ability, neat and tidy but PDQ. We had another particularly technical phrase too, that indicated a greater emphasis on speed. And that was PDFQ. However, this was usually reserved for the boss of the workshop!