Who knows what the regulations are for a Soviet warplane brought into the US but for most planes the owners are not able to just machine new parts unless the plane is under an experimental certification. A lot of home built planes are under experimental certification. Civilian factory built planes and US warbirds are not and so they can't just machine new parts freely or the airworthiness certificate is at risk.

The regulations go on forever. If you make new parts you risk having to re-certify the plane which makes an expensive and difficult project get much harder.