I have a Regina America freewheel with a broken cog I've been looking to replace. It's a 13-tooth cog in the second position (labeled "D1"). Problem is that I haven't seen many individual Regina cogs for sale, and not the one I need. There are, however, lots of complete Regina freewheels of various models available online, at auction, etc. So, I'm wondering if I were to buy a freewheel of another model (such as CX, Extra, Oro), would I be able to interchange the cogs--assuming I'm going from a 6-speed to a 6-speed? If not the low-end threaded cogs, what about the unthreaded high end cogs? I could use a bit higher gear at the other end as well if I wind up with some extra cogs. Thanks in advance for your guidance!