I think by the time you have Mechanics and Fabrication up near 10, the only real problem is “I don’t have a machine shop” - CNC tools, metal lathes, etc.
These things DO exist in America, in New England, despite the best efforts of the corporatocracy to make them extinct by offshoring all the jobs that use them.
For the likes of, say, a big-block, brute-force dumb muscle car engine (such as the legendary 426 Hemi,) it primarily comes down the lack of appropriate tools; and appropriate tools are something a survivor ought to be able to build, with lesser tools but exceptional skill - or to loot, if they feel like locating a machine shop and clearing it of zombonis. After that, the only real issue is the lack of power, and the lack of a building power grid. Both of those could be “simply” solved the nasty, hacky way of simply making them a vehicular tool (ala vehicular welders,) mounting them on a cart with a storage battery and swapping with a battery charged by a solar power station out front.