There are a decent number of requirements placed on manufacturers, but the use of vehicles in the US are primarily regulated at the state level. Most states do not perform safety inspections vehicles, and even when they do, they are minimal in nature (e.g. do the lights work, are there holes in the exhaust, are the tires and brakes worn out?)
Police can pull someone over for faulty equipment, but that would generally have to be an egregiously obvious defect.
From a federal level, the primary thing that is illegal to modify is emissions equipment (however, if a vehicle is registered in a state without inspections, even that is essentially unenforced)
In much of the US, you can legally drive just about anything with brakes, seatbelts, lights, wipers, turn signals.
https://en.wikipedia.org/wiki/Vehicle_inspection_in_the_Unit...
Police can pull someone over for faulty equipment, but that would generally have to be an egregiously obvious defect.
From a federal level, the primary thing that is illegal to modify is emissions equipment (however, if a vehicle is registered in a state without inspections, even that is essentially unenforced)
In much of the US, you can legally drive just about anything with brakes, seatbelts, lights, wipers, turn signals.