Upgrading the stock cartridge on Macintosh’s MT5 turntable

anyone?

compliance of cartridge is important to start choosing compatible options. to get the tonearm resonance (they all have resonance) in the correct range.

https://www.ortofon.com/support/support-hifi/resonance-frequency/

find the total weight of the arm/headshell, you can ask McIntosh for tonearm answer, and their recommended range of cartridge compliance. you can also find what the compliance of the existing cartridge is, use that as guidance. factory headshell or alternate? same weight as original?
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estimated hours on stylus?

your alignment skills very good? eyesight? alignment tools?

any chance stylus suspension has changed slightly, altering your VTA? You could try raising the back of the arm very slightly, listen for original nirvana. I just 'fixed' my friend's Grace Arm/Cartridge, his stylus cantilever is bent. He will need to have the Stylus replaced, but it got back to quite listenable.

Wow.  Thank you for the direction and questions.  I am NOT adept at cartridge alignment.  It was done by the installation crew when I bought the tt.  I am guessing I have an avg. of two hours/wk. * 7 years of listening time.  I will contact Macintosh for the information.  The tt is stock and has not been touched in terms of changes...

Running the risk to get some flak saying that this arm-cartridge resonance it for many applications somewhat overstated...
It took me some time to come to this conclusion. 
My best guess though would be that your tt's arm, being only some 7 odd years old, would be off medium effective mass, somewhere between 10—12 gram effective mass, as most more current tonearms would be. 
Given that assumption to be quite correct, any cartridge with a compliance ranging from about 15 to 20 cu (compliance units) would be pretty well suited as an replacement.
In the price range of around $ 500 — $ 1500, replacing your current item, in my estimate, practically all new stock cartridges will be in this range.
So now the more important issue I'd see is whether you go for an MC or MM cartridge. 

What I perceive in your situation, as you have eluded to, I'd HIGHLY recommend a good MM cartridge - though NOT necessarily an item such as an Ortofon 2M Black.... as yet. 
It's a top performer but maybe a bit to tricky and can sound somewhat to bright in a number of situation. 
Audio Technica has a large range of current ones to choose from, Ortofon, Nagaoka etc.
All this is no rocket science. 
The issue with numerous NOS (new old stock) cartridges is that they quite often, in my experience, proof more tricky to set up, as well the tendency having been designed in the majority for more light-effective-mass tonearms i.e. of compliances 20 cu and some easily as high as 40 cu. (soft as butter in the sun ☀)
Depending on your setup, those could work fine too, I have some, but again not what I'd recommend. At all. 
I hope this will give you some more ideas. 😉
M. 🇿🇦