Sun System Handbook to the rescue.
http://sunsolve.sun.com/handbook_pub/Systems/Ultra20/Ultra20.html
The Ultra-20 is a currently active and shipping model.
Glance through the Full Components' List that's linked on that SSH page.
There are a number of qualified processors.
Socket 939 form factor.
Keep a couple of things in mind:
There may be quirks and idiosyncrasies regarding any cpu upgrade.
Go through every bit of the online system documentation shown from the SSH.
Better yet, if you have a login account with SunSolve,
use the Spectrum version of the SSH, to see more details.
[url=http://sunsolve.sun.com/handbook_private/Systems/Ultra20/Ultra20.html]( Ultra-20 in the Sun System Handbook after login )[/url]
And ...If there is any sort of warranty or service contract coverage on that box,
you will need to use Sun parts if you hope to maintain active coverage
for that warranty or contract. Parts from third-party sellers may invalidate
or suspend the coverage for as long as they are in the chassis.