SPROCKET RATIO w/ 21" REAR WHEEL!!!!!

I have a 21 x 3.25 wheel with a 120/70/21 Metzeler tire I want to use with a sprotor setup (single flange hub). Sprotors I've looked at come either 48t or 51tooth. JT front sprockets for my 1979 kz650D2 come up to 18t. So my question to you guys is what would be an acceptable final drive ratio with this tire (28.5" dia) using any combination of these sprockets? Or can I get a custom front sprocket somewhere if the desired ratio is not available with common sizes?

www.gearingcommander.com is a website to use when figuring out ratio's,chain speeds and factors in tire sizes too.