I used a polini evo piston, in my old Firefox. I Also had 10mm.
Big difference between malossi and polini pistons is that the evo piston spring is harder than a mhr spring.
Example.
In my Firefox with original evo cilinder & piston. I drove 7000km with good compression.
With the mhr piston in the evo cilinder I lost compression after 2000km.
After a while I changed the cilinder 2 a normal mhr cilinder..
Compression was lost after 1500km.
After changing the pistonrings 2 a polini evo pistonring, I drove more than 3000 km with them..
Firefox setup was.
Polini evo 1 lc cilinder, later mhr lc cilinder
Mhr exhaust.
21mm dellorto
Malossi overrange kit.
Malossi mhr deltaclutch.
Mhr multivar.
I used the scooter as a daily driver..