I understand what you want but not what you already have/ will need. You will at least need some kind of way to make the gear fixed and not just a freewheel that can rotate. It can be difficult if you are trying to stick with the same hubs/ wheels. you will also need a chain break to shorten your chain and even if it ur chain is no good you still may need one to put your new chain on. Does it have to be fixed? or could it just be singlespeed? i ask because fixed means constant peddaling and all your inertia is in the rotating crank so it is kind of scary to ride at first when your realize how much energy they are spinning with and how hard it is to stop or skid.