Porkchop sell 20 x 1 3/4 tyres and have this disclaimer:
* Size: 20" X 1 3/4" (IMPORTANT: These only fit Schwinn S-7 rims... they will NOT fit a 20" X 1.75". This is because a S-7 rim has a 419mm bead diameter and a "regular" 20" X 1.75" or 2.125" rim has a 406mm bead diameter. Although mathmatically the number looks the same, in the world of tire sizing, it is a COMPLETELY DIFFERENT SIZED TIRE. However, 20" X 1.75" TUBES are compatible with these tires.)