Well from what I gathered when a friend of mine decided to go crazy moding his MR2 with regards to engine work.
The vehicle should not have any foreign objects sticking out of the bonnet, tyres should stay withing guards, if tyres and diff size increase to outside the body, flaired guards need to be created to enclose these things.
Bonnet Scoops: (letterbox type) to a degree are illegal.
Exhaust & Emissions: Must meet EPA regulations/emittions.
Engineeres certificates can vary from state to state...
Same with mod laws...