Well since chains are based on tensile strength, I suppose you could go to a different size if you could find a match, but I'd advise against it. It's hard enough finding aftermarket sprockets that fit our bike with the 525 setup as it is.
For the last month I've going round and round with Sprocket Specialists trying to get a rear sprocket that fits. After ordering one off their application chart for a Speedmaster(pn 2696S) that didn't fit, I'm now waiting on one that fits an America(pn 2703), but they told the retailer I ordered it from that it doesn't come in 42 tooth, and they'd have to scrounge up a blank to cut one and it'll be another 2-3 weeks. This is the 3rd rear sprocket I've ordered. The 2nd one was from JT Sprockets and it didn't fit either.
I can't imagine what it'd be like if I tried to get a rear sprocket to fit in a different chain size like a 520. So next time, I'll just go with Brent, or just order the Triumph OEM kit from triumphestore.com. Their price is something like $189 which includes both sprockets and the chain. Local Triumph dealer price is about $40 more than triumphestore.
