Most or all of the freewheels I've had apart have different size ID's for the various cogs. The smallest cog attachment varies but it's usually just threaded onto the body to hold the rest of the cogs. You'll need to use the smallest cog that came with the freewheel or maybe a BB lockring or the smallest cog with the teeth ground off to hold it all together. The spacers are also different sizes to fit over the stepped-size freewheel body so it will take a few donor freewheels to make one 2 speed.