As I see it you have 3 options with this, the fist 2 will never be great for 2 up riding.
Pinasco 102 but the gearing will most likely be too short with that.
Polini 75 HP kit but it will also mean buying the suitable cylinder studs, which run from the case fully through the barrel as with 125 fitment, and also the bespoke cooling cowl/ shroud. Very large cooling fins to allow for a massive cooling area. But very expensive all told.
A standard 125 engine to transplant into your scooter. This would be your best option in my opinion.
Black Widdas S.C. - Taking idiocy seriously since 1999.