I'm in Pennsylvania (and ride a Vstar with a Voyager Conversion registered and inspected as a motorcycle) and this is the current definition of a Motorcycle:
"Motorcycle." A motor vehicle having a seat or saddle for the use of the rider and:
(1) designed to travel on not more than three wheels in contact with the ground; or
(2) designed to travel on two wheels in contact with the ground which is modified by the addition of two stabilizing wheels on the rear of the motor vehicle.
The term includes an autocycle.
Then again, the Hannigan 4 wheeler might be this .... also in the Pennsylvania code under Vehicles... LOL.
"Human organ delivery vehicle." Any vehicle which is used or intended to be used and is maintained or operated for the purpose of transporting human organs or human tissue on an emergency basis.
Here's the link if you want to look:
https://www.legis.state.pa.us/WU01/L...1.002.000..HTM
EDIT:
I might add that there is a reciprocity law that would let you use motor vehicles legally registered in other states. That's why you don't get pulled over with out of state plates if your vehicle doesn't meet registration requirements. Safety? Yup, probably going to talk to the nice man in the car with flashing lights.