Massachusetts RMV says that a moped is a "motorized bicycle."
In order to be classified as a moped, it has to have a cylinder capacity
of no more than 50 cubic centimeters and an automatic transmission.
It has to be capable of a maximum speed of no more than
30 miles per hour. Naturally it has to comply with all applicable
federal motor vehicle safety standards.
   You have to have a moped sticker, issued by the RMV,
and it has to be affixed to the moped.
The sticker won't be issued unless it meets the above requirements.
   Moped operators are subject to the traffic laws, rules and
regulations of the Commonwealth of Massachusetts.
Mopeds have the right to use all public ways in Massachusetts,
except limited access or express state highways where signs
prohibiting bicycles have been posted. 
Mopeds may use bicycle lanes next to various ways
but are excluded from off street recreational paths.
Moped operators must signal their intention to stop or turn by using either hand.
Mopeds may keep right when passing a motor vehicle
which is in the travel lane of a way.

More regulations:
Mopeds will not be operated: By any person under 16 years of age.
or by any person who does not have a valid license or permit.
or at a speed greater than 25 miles per hour.
or without the operator and any passenger
wearing a DOT standard helmet.

For More info on the federal level sources differ but here is the essence of it all.
Motorcycle engine size requirements for highway driving.
"Brake horsepower" (BHP) is the rating for highway use.
6 bhp is the legal requirement for access to interstate and limited access highways.
Modern 150cc and over can usually meet this requirement.
Some old bikes may not.
But! If you are cruising along the highway on a 150cc bike at 40MPH don't be
surprized if you end up having a nice conversation with the state police.

Most importantly the question would be if the motorcycle can handle
your weight when in aggressive traffic situations such as
highway speeds and lane changing, passing, etc.
Can it keep up with regular highway traffic?

I had a 2 cyclinder 440 cc bike that "felt" too light on the highway with
my 250 .lb body on it let alone a passenger. It felt like I was on a bicycle.
I wish I still had it for backroads though!

You have to be able to get out of the way of a situation
with a bit of additional power but if you are at the extent of your power,
you can't speed up past a "situation".
It was nerve wracking to be so vulnerable.
My 1100cc goldwing is much more stable as is my 750cc Suzuki.
Essentially , if you don't have enough power then you don't have enough control.

Can it brake well?
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California gov says:
"A motorcycle is a vehicle whose motor displaces more than
150 cubic centimeters and has a seat or saddle for the use of the rider.
It is designed to travel on not more than three wheels in contact
with the ground and weighs less than 1,500 pounds."
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"A motor-driven cycle is a motorcycle whose motor
displaces less than 150 cubic centimeters."
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"A moped or motorized bicycle"

There are two types of motorized bicycles, defined in the California Vehicle Code

(VC) Sections 406(a) and 406(b). Section 406(a) VC refers to a moped
or motorized bicycle as any two or three wheeled device having fully
operative pedals for propulsion by human power, or having no pedals
if powered solely by electrical energy, has an automatic transmission,
and a motor which produces less than 2 gross brake horespower
and is capable of propelling the device at a maximum speed of not more
than 30 miles per hour on level ground.

Section 406(b) VC refers to a motorized bicycle as a device that has
fully operative pedals for propulsion by human power and has an
electric motor that:

• Has a power output of not more than 1,000 watts;
• Is incapable of propelling the device at a speed of more
  than 20 miles per hour on level ground; and
• Is incapable of further increasing the speed of the device
  when human power is used to propel the motorized bicycle
  faster than 20 miles per hour.