Welcome aroy! 'Nother Mainer here
The laws in Maine regarding motorized bicycles are somewhat vague at best: Maine Bureau of Motor Vehicles
to actually have a specific entry for motorized bicycles - the usual parameters regarding such things, but it disappeared last month or so. I've heard some rumors (good ones) but they're just rumors ofc.
Currently the way the site is written you may at first mistake the scooter entry for what yer lookin' for - but obv that doesn't work with the "Each wheel (not tire) is less than 10" in diameter" let alone the engine size restriction. The only remaining entry referencing motorized bikes is this byline in scooters: "Can be used on a public way like
a motorized bicycle or tricycle"
I have chosen to interpret that (with the info that used
to be there) as MB's are legal if they stay under 20mph (pfft sure lol), have 3 wheels or less, and while you don't need registration or insurance you do need a operator's license (any sort) to have 'em on the road.
This unfortunately could be a problem for your 4 wheel idea
Please note this is an interpretation
on my part and also that it includes information no longer available on that site. While I did get pulled over and the officer "checked it out" and had no further problems with me - I'd still make a few calls to see if you can't get some better info. Remember to make a coupla calls tho as you'll very likely get vastly different answers every time.
The regulations that used
to be there and most likely still apply;
A motorized bicycle/tricycle is a vehicle that has a motor attached to a wheel and is no more than 1.5 brake horsepower and has a cylinder capacity* capable of propelling the vehicle unassisted at a speed of 25 m.p.h. or less on a level road surface. A motorized bicycle/tricycle may have foot pedals**.
* An operator's license or a motorcycle, motor-driven cycle or moped endorsement or restriction is required.
* Registration is not required.
* The operator of a motorized bicycle/tricycle must ride to the right, except when making a left turn.
* The vehicle may not be operated in excess of 20 m.p.h***.
* An operator may not ride other than astride a regular and permanently attached seat.
* May not be used to carry more persons than the number for which it is designed.
* A person may not attach the vehicle to a moving vehicle on a public way.
* Operator may travel on a paved shoulder.
* A municipality may make other provisions for the location of motorized bicycle/tricycle traffic.
* Operator of a motorized bicycle/tricycle must obey all other traffic laws.
The "*" is me btw and this is why;
*note that here
no maximum displacement is mandated beyond "capability"...
**note that a motorized bicycle may
have pedals... so this means "may not" as well?
***may not be operated
in excess of 20mph, which ofc is different than the "capable of 25
mph or less" stated earlier.