It seems that the usual condition for an aircraft PSRU is to fail. It appears to be difficult to design one (which is incredible, because similar speed reduction units are widely used in industry elsewhere and I am quite sure that there are established ways to design them properly).
I bumped into this manufacturer about which somebody flying a Ford engine was very happy about. So I decided to share the link if that interests you any.
Not sure if the price-what you get ratio is anywhere one could call affordable (7500 per unit) taking in account that this is a chain drive unit rather than a sophisticated reduction gear. However, reportedly this works. And the web page says at the moment "PSRUs are temporarily unavailable ". Maybe this is temporary I hope.