First, I find it hard to believe that swapping from an 11 to a 12 tooth cog causes all that much strife, but the legs want what they want. Components - Drive Train - Cassettes - 7 speed - Niagara Cycle <---- here is a link to a page with all sorts of 7 speed cassettes for sale, from a company that generally knows the difference between 7 speed and 8 speed. Any one of them listed with an 11 tooth small cog will get you back to where you want to be. Beware the one cassette on that page with a 10 tooth cog - I don't know if that would be compatible with any old hub/chain combo. Getting a minor change in gearing does not generally require disassembling hubs or building wheels.