If an 11T clutch is too hard to find, or overly expensive, you could go with the common 12T for a 6:1 ratio. I think that ratio would be too low when combined with your 15" OD tire but you can always change a jackshaft sprocket if need be. The less-common tooth count clutches tend to cost a lot more than the common ones, you may be able to buy a 12T clutch ($40 or less for a Max-Torque SS) and a jackshaft sprocket (probably $10-12) for less money than an 11T clutch. I prefer odd-number tooth count sprockets because they provide reduced chain and sprocket wear compared to even-number sprockets (was fairly common knowledge among motorcycle enthusiasts many years ago, requires a somewhat complicated explanation that I won't get into here) but you can't always get what you want, at least not at a reasonable price. I was looking for a 3/4", #35, 13T clutch a while back and saw some crazy prices before I found a Max-Torque SS for under $40 on eBay.