Well, I got some parts, but my car is still not fixed yet so I can't test them just yet...
Black is PETG, white is alloy 910, which the guy I found on 3dhubs gave me as a discounted sample. The PETG is dirt cheap and more than strong enough, so I am not bothering with nylon or 910 anymore.
This will be 11% underdrive (so 89% of original rpm). I will need to make sure the belt actually fits, I may have gotten too ambitious and the tensioner might hit the pulley like this.