LMS 5100 7x16" drive belt ID

[Spajo]

I have a LMS 5100 7x16" lathe and I want to change either the motor pulley or the spindle pulley or both. My goal is to give up RPM for a bit more torque. I prefer to change the motor pulley. That would be simple to do. So, I just want to know how to shorter (or longer), but compatible belt. The specifications that are give are:
100 teeth
9 mm wide
2.25 mm height (thickness ?)

I they don't have a length.

Thanks
Spajo