The sewing is not hard. Position your covers were you want them and use clothes pins to hold them in place. Use an awl to make the holes. With a needle on both ends of your thread start next to a leg, pull a needle through the first hole to the middle of the thread. Go to the next hole and push one ...
Check the firing pin hole in the bolt head. If it is not supporting the primer you could have piercings even with the correct protrusion. Gas erosion will tend to open/bevel the hole. Your project is looking great!
If the plate is mounted square and proper take a small drill bit and drill a couple of alignment holes for reattachment before removal. After your machine work use the same size bits to locate the plate for welding. Weld the holes up after you are done and grind flush.
The long rod on the end of the trigger can be fragile. Also with the welded in trigger stop there would have been no way to remove the trigger. This way all of the internals can be changed for repair without grinding and welding.