Simple answer is that there aint none - sorry. You have to use a 120/90-18 and modify the front bracket to fit it. Some folks have ground the middle of the bracket off (which changes it from a fork brace to a fender bracket) and others have extended it upwards slightly.
If you're really enthusiastic you can get a Thunderbird Sport front wheel which is 1" smaller and will take a greater range of tyres... this also gives you twin front disks if you want to go out and get the extra speedmaster bits to put a second caliper on.
Matt
