once again... you gotta talk to the right people and get the sprockets yourself. you only need two 1/2" pitch sprockets.
4SR 08B-1 (DUNLOP) Steel Chain Sprocket 1/2" Pitch BS Bore - 8T to 38T | eBay
that listing is not cheap! i know my local sprocket place does them for at least 1/3 of this price. a pair of these, then find someone with a lathe and a welder.
these guys are in kiwi land...
British Sprockets Sprockets ::. Sprockets NZ
08B is 1/2" in the british sprockets.
you just gotta make some phonecalls and price hunting...
you dont go to a place and ask them to make the whole sprocket assembly, that means hobbing out teeth and machining the whole assembly. you just get two sprockets welded (or bolted) together, and bored out for bearings. its NZ! nearly every farmer out there has a lathe in his shed!
crap, even i would do it for a bottle of southern comfort (cus the NZ stuff is far superior to our cruddy aussie muck!) but i am not sure how you would get it to me
you also get to play with different sprocket sizes. doesnt matter how many teeth you use. if you use identical sprockets, the ratio stays the same regardless of the number of teeth. but if you get two sprockets of different sizes, the ratios will change a bit. ie, a 19 and a 21 tooth will change ratio. two 20 tooth will not.