When restoring vintage aircraft can you legally build engine parts from scratch when none are available and what are the guidelines if yes?
Short answer, no.
If it's for static display, you can do whatever you want. If
it's going to remain airworthy, the parts need to be both correct
and certified as airworthy. At least in Canada...
Uncertified(non-aviation) parts have brought down many aircraft,
especially automotive hose clamps.