Well, after 12 years, my 4hp HSD spindle finally said "Enough"!
Instead of getting another one, to the tune of $1800 for a bare spindle, I decided to give AliExpress a try and try one of the Chinese clones. This is a HQD 3.5kw (4.7hp) air cooled square body one that bolted right up to the Z without modification. It was originally pictured on AliExpress as having the same plug as the HSD, but came with the connection as shown, so I had to cut off the cord and wire it in. No big deal, just not as convenient as it could have been. For $350, I can live with it. So far it's working nicely and the noise from the shaft driven fan is not bad at all. It's running at 12000 rpm in the video. Phone mic doesn't reflect the real sound, but it's 70db at 2 feet from it. Way better than the old one in it's last days!
https://youtu.be/xKSPhmZdbZc