Rebuild or new cartridge with my zeta tonearm

I've just researched the heck out of MM, MI, MCHO, MCLO, SUT, MC pre-preamps.

Many think rebuilding is to save money, however you have many other important reasons to rebuild that Sumiko. If you do, find the best to do it, pay what you have to, continue to love it!

I found, the channel separation and tight channel balance of most cartridges, any type, only becomes greater/tighter in the darn expensive range.

Your Sumiko has both excellent separation 30db and tight balance 0.5db. That is excellent, so, I suggest you rebuild it, or, don't go down in specs if you move to a new cartridge.

As it happens, I just left my beloved Shure V15VxMR with new boron/SAS stylus from Jico, to my first MC using SUT into my equally beloved tube preamp MM phono. The MC I chose, AT33PTG/II has same 30 db separation and 0.5 balance specs as your Sumiko.
After a few weeks now, I am certain it makes a more solid center, and a wider image, it won, the Shure went upstairs for a new competition in my office.

Remember, there is no center signal/reproducer, it is a Phantom, created by EQUAL left and right. 

You also have the advantage of tracking 1.8g, my new AT wants 2.0 g; I prefer Shure's 1.25 tracking, however ...

If yours has enough volume now, then you skip the cost and extra device/cables/hum possibility of MCLO, 

My only concern on the existing Sumiko is that it may just be a tad light for the Zeta.
It was originally about $850 new, not sure if I would spend $500 to $600 on a rebuild.
It will likely not have the same boron cantilever on return.
Now it is entirely possible that the OP may really like the sound of the rebuilt cart but is an expensive gamble IMHO.

My only concern on the existing Sumiko is that it may just be a tad light for the Zeta.
It was originally about $850 new, not sure if I would spend $500 to $600 on a rebuild.
It will likely not have the same boron cantilever on return.
Now it is entirely possible that the OP may really like the sound of the rebuilt cart but is an expensive gamble IMHO.

Lyra Delos

tzh, the Delos is well over birdfan's budget. Birdfan, All of my cartridges save one are LOMC. I do believe, actually I know you would be better off with a MM cartridges than an inexpensive MC cartridge. The Clearaudio Performer V2 fits the bill perfectly. It is $450 and is a good match for your arm. It tracks at 80 uM which is excellent. It will be more dynamic than any LOMC in your price range. Warning, stay away from vintage MM cartridges. They are all way too compliant for your arm. The Grados and High output Soundsmiths are borderline. You could use a low output Soundsmith  but they are about your price range.