Shelter 501 mkii then what now

If it needs re-tipping you really have nothing to lose. You end up with a great sounding cartridge for $250 - $350. It sounds different, but not necessarily better or worse. Hard for me to describe with the passing of time since I have heard it.

tracking ability improves drastically to the point of producing substantially less surface noise. Ruby rings substantially less and has less resonance vs. aluminium or boron.
I've no doubt that 901 may sound better, but you won't loose a dime if you just upgrade current.

I like the idea of retipping, but on a Japanese cartridge would prefer not to use the European stylii that Soundsmith typically use (VanDen Hul, Gyger).

I would prefer the Japanese stylii (Namiki, Ogura) and for that Andy Chong at Needle Clinic is the go-to guy:

http://www.phonocartridgeretipping.com/contact.html

Best of luck in whatever route you take.

I have only used a SS Level 2 stylus on my 501 since buying it used: under a microscope it the original stylus showed a lot of wear and the seller kindly and immediately paid for a retip. Pretty cool.

So: the Level 2 is revelatory for me, particularly against the elliptical styli Shelter likes to use. It simply gets into areas of the groove that haven't been played before, and reawakens a lot of older records I'd written off. I really am an adherent of the Level 2 upgrade because its profile isn't as severe and difficult to line up as the Shibata Level 3 tip. Hope this helps.