Google the chain pitch thing.
Visit a mower shop and get a bit nosey in and around and under some rideons. Theres a few different ways they do things.
Im not sure about how good your info would be to someone who had what you were looking for. Probably good enough i would guess. I was just making an observation.
Short of buying a parts mower, you might be waiting a while before someone shows up with the bit you want but then you just never know.
You can buy sprockets with bearings in the centre.
And you can buy pulleys (Vee belt, flat belt) with bearings in the centre. Some have straight holes some have offset holes, they are designed to work as tensioners. Cars use them for fan belts and air con compressors etc.
Obviously a belt pulley will not work very well with a chain. It will be noisy and wont last long.
