Any reason to do a stylus and not just a new cart? Newb at this obviously.
A little late but anyway:
A MM cartridges only component that "wears out" is the stylus - the "generator" has basically an unlimited life. So replacing the stylus, provides you with a "brand new" cartridge.
Replacing the stylus, can also be done without in any way changing the alignment of the cartridge/arm - so you avoid all that geometric messing about....but...
The key thing to a cartridges performance is ultimately the cantilever - it has a greater impact on the "sound" than almost anything else - it's mass will drive the effective moving mass of the whole system, it's rigidity will drive the flexing and associated distortions, etc...
The cartridge itself, and whether it is MM or MC or MI or whatever has a lesser influence.
The needle type and shape (and quality) - has an important influence on the ability to properly "read" worn records, as well as the lifetime of the needle (a line contact design will have more than double the effective lifetime than a conical/spherical or eliptical stylus)
The Shure family bodies came in 3 basic groups - each of which had for the most part interchangeable styli within their group...
The Square bottom group (M70 family), the V Bottom mass market group (M97 family) and the premium V bottom group (V15V family)
The M97 and V15V groups look very similar, but the stylus "shank" - the bit that gets inserted in the body - is much narrower, more precise... for the V15V's - the M97's have a "fatter" shank.
The V15V family use laminated poles on the internals of their generator bodies, which reduces eddy current associated distortion.
The M97 family use standard (unlaminated) poles
Currently the best stylus you can get for any of the Shure bodies is the Jico SAS - an exotic low mass high rigidity cantilever, with a very extended line contact needle on the end.
To get an idea of the comparative performance potential - you should do a search for other cartridges that combine a Boron, Ruby, or Saphire cantilever with a line contact needle ..... the Jico needle remains excellent value for money.
Having said that - performance can differ quite substantially between needles - due to cantilever differences - and that is why you will also find that within those generator families there is quite some variation between the different models in terms of their inductance and impedance/resistance.
On a high inductance design (AKA: MM) - the circuit formed by the resistive and capacitive load, along with the inductance, provides a substantial EQ effect, which combines with the cantilever behaviour to (in theory, on a properly designed complete cartridge system) - provide a reasonably flat frequency response.
When using a non-original stylus - you therefore potentially alter the voicing of the cartridge substantially - given that you maintain the "standard" loading.
Something that is often overlooked by many people is that the manufacturers were intentionally vague with their loading recommendations - Shure typically suggested an R load of "up to 70k ohm" and a wide range of capacitance values as well.
Varying R & C with a given cartridge, can completely change its voicing.
One of the joys and pains of using MM's - is this wide variability.
Now that original styli are long gone for many of these cartridges, the voicing of the cartridge system is up to the individual.
By frequenting ebay and similar places, you can often pick up old Shure bodies from the V bottom family, and build up a small set of bodies with differing inductances (variable 1)...
Getting an MM phono stage with adjustable loading is key to getting them right.... R should be able to range from around 20K to 70k ohm, and there should be as little capacitance in the system as possible so as to allow for adjustment of total C from around 100pf to 500pf.
Typically an MM phono stage with spare RCA sockets for loading plugs is the easiest way - you can then make up "loading plugs" for various R & C loads...
The other thing of interest that most people don't realise, is that there is substantial manufacturing variation on styli even from the same manufacturer... and that the loading adjustment allows one to compensate for this variation.
With MC's there is very little ability to EQ / Adjust / Voice the setup - you get what you get - and you get the manufacturers voicing (for better, or worse)
For the most part that means an MC will sound like it's cantilever - and with the very best cantilevers - that implies a flat frequency response (as the resonances, distortions are pushed into the 30kHz+ frequency range, outside the audible range) - but for anything with a standard aluminium cantilever, and even the heavier versions of the ruby/saphire/boron cantilevers, the resonance frequencies will be in the 12kHz to 19kHz range - and will affect the sound (will to some degree become the "signature" of that cartridge!).
I have examples of most of the range of V bottom M series and V15 series cartridges.... I recommend them highly, in combination with the Jico SAS.