With the vertical shaft motor in a dirty environment and grass the alloy pulley may prematurely wear out ,I've seen alloy pulleys on some self drive mowers but the crank and v belt is completely sealed behind a plastic cover even then most are steel pulleys.
The grub screw doesn't necessarily need a flat spot to stop it spinning it may be easier just to drill a small locator hole in the boss for the screw to lock into.
I've used old VW pulleys and just made a spacer in the centre so a wider V belt fits and welded the spacer in the centre so it can be machined out to fit a shaft .If you could get VW pulleys for around $16. each it might be another option.
It was always quicker welding one together than machining up the complete part. A second hand water pump pulley at a wreckers is about $17. each.
