This web site gives these specifications for the motor:
Bore (in) 1.54
Bore (mm) 39
Compression Ratio 7.21
Cooling Air
Cylinders 1
Displacement (cc) 49.5
Displacement (ci) 3
Engine Configuration Single-Cylinder
Engine Immobilizer Not Available
Engine Type 2-Stroke
Fuel Injector No
Fuel Requirements Regular
Fuel System Type Carburetor
Number Of Carburetors 1
Speed Governor No
Starter Electric / Kick
Stroke (in) 1.63
Stroke (mm) 41.4
Valve Configuration Reed Valve
It appears from the specs that the differences are between the People 50/150/250 models, not amongst the models themselves. The 150 and 250 use the four stroke motor, while the 50 is a 2-stroke. I'm pretty sure there is only a single 2-stroke motor which is used in the People 50 and not multiple different engine models.
Malossi makes a big bore cylinder kit for your scooter. This is just a suggestion, not an endorsement.