Keep in mind that the shaft must be hardened as much as possible as the lower bearing is a needle bearing riding directly on the shaft (not the best concept IMO but oh well). See here all the trouble I had to go through to fix those shafts (they are NLA) on my deck as the bearings haven't been replaced when they should have
Repair to MF 1655 mower shafts
The alternative might be to bore-out the mower spindle castings to insert a bigger needle bearing using a sleeve on the shaft you will sell but that requires quite a lof of modifications. You could use the same size needle bearing with a reduced shank on the shaft at the needle bearing height but you would have to use a new way of getting the blades to turn as the "double square" drive of the blade holder isn't engaging much on the square of the 1" shaft therefore a smaller shaft couldn't be used.
Those mowers aren't of the simplest concept nor easiest maintenance since but damn they work amazingly well. I think building a custom broach, even if out of standard steel would be the simplest and suit your needs to machine the POM, especially since you were able to go through all the hassle of machining the cogs, which I find amazing. That would really be a better-than-original solution that your customers could use straight out of the box.
I think the main question is: are you equipped to machine the shape of the splines in a steel shaft?