It seems like this is the 10th time that I've seen this same question... Build it... I'll buy one! It seems that one issue is that your average pressure washer pump runs significantly faster than 540rpm... so you'll need some belts & pulleys.
If you don't mind going to England to get one, there's <A target="_blank" HREF=http://www.lloyd-jones.com/store/cleaning/pto3030a.html>this one</A>.
Well, at least somebody makes them. The tough part may be finding one.