As far as Federal law is concerned....
There is no Federal Firearms License needed to work on your own guns.
If you intend to work on firearms that belong to others it would be prudent to get an FFL.
You can modify, alter, change, repair, etc ANYTHING on your own gun as long as you do not violate the provisions of the National Fiream Act or the Gun Control Act.
This means no conversion to full auto or shortening the barrel or overall length below what Federal law requires. If you are refinishing the gun be sure to not obscure or obliterate the serial number, manufacturer or model markings.