Forgot the Alfine, shame on me…
Edit:
Yes of course you are right.
I mean there’s no chance to get a custom belt.
All you can do is try to find an 80 teeth wheel and matching crankset.
The belt is not a problem for Schlumpf/ATS drives.
Maybe you need spacers for the beltwheel.
Edit II:
Chain drive is not an option for you?
Maybe one could use 62:20 or 58:19 