How about taking the old o-ring to the parts store and finding one that matches? That's what I did.
In fact, I have a spare for a 1.8 I can bring you or we can meet up somewhere. Not sure if the 1.6 is different.
If you can't wait the o-ring I have has the following measurements.
W - 1/8"
I.D. - 1-1/2"
O.D. - 1-3/4"