The latest version of the 125 water-cooled motor is very similar to the air-cooled Typhoon and Skipper motor and the water-cooled Hexagon, with minor improvements to several parts of the motor. But all the things that apply to the air-cooled and older water-cooled motors still apply.
It is possible to modify the original 125 cylinder and head, fit the 150 Piaggio cylinder and head or to just fit the Malossi 172 kit cylinder and head. The Piaggio 180 cylinder does not fit the 125 crankcases.
The 125 transmission is similar to the Typhoon and Hexagon motors. It uses the same gear ratio but the spline fitting is different. Because of the 12" tyres the overall gearing is higher. Malossi make a gear kit with the different spline for the 125.

A standard 125 gives 12bhp
A standard 150 gives 13bhp
A modified 125 gives 14bhp
A 172 Malossi kit gives 17.5bhp
A 180 motor gives 16.5bhp

(The above power figures are at the rear wheel with standard carb and exhaust.)

Standard 125 v. Modded 125 v. Malossi 172 Cylinder & Gear Kit
(green, blue and red respectively)