Hello!
Mechanics don't have their own distinctive uniforms, do they? I'd go for a civilian look - overalls or something like that, maybe an army field cap if italians had something like that. But even that wouldn't be practical - if you crawl under the car the cap is sure to get dirty anyhow - that's trouble from the sarge, to be sure. So I'd go for something dirty looking and civilian. It could also pay off to look for pictures of a mechanic from the thirties, forties and fifties - references are always a good thing to look at. Good luck with your build and have a nice day
Paweł