Street legal for most states in U.S. except Cali: Working head lights, brake lights, turn signals. Has mufflers and glass window(s). Also tires with DOT tread.
For the most part that's the basic's you need to remain street legal in the U.S. and pass most state inspections. Most states are leanent, and everyone know's a guy at a local garage who will give you a state inspection pass if there's something on there not quite legal. There really is no cap on HP, what engine modifications that can be done, as long as you can drive it and not kill yourself.