I bet SOMEONE in the '70s tried to modify their car's parts to try to make it go faster, but didn't bother buying nicer replacements for said parts. Perhaps we add a crafting-menu-thing from which to attempt to modify or build car mods? Some mods would simply be "grind excess bits off of XYZ part", which would increase performance slightly at the cost of part strength (not what state the part is in, but how quickly it'll wear down), whereas others would be complex, like "Build homemade blower" (the resulting part would be ~$100 cheaper than a manufactured one, but would perform 15% worse, would wear out more quickly, and would be 25% heavier), or "remove emissions controls", which would make the car noisier and slightly faster at the cost of having a chance to be pulled over by police.
Thoughts?