If you liked how it sounded, its probably an appropriate cartridge. The Miracord arm is probably somewhat massier than ideal for the M91ed, but I believe that reality is a good deal less hard and fast than theory, and plenty of people love how M 91ed and V15 and other high compliance cartridges sound in Miracords and Dual 1019s and so on, and its hard to argue with success.
But: in its heyday, the M91ed could be bought from every stereo store in the country for $14.95. Now replacement stylii cost a lot more than that, and there are questions about how good the modern products are. So I would hesitate to pay a lot for a replacement stylus.
Here's my take: the cheapest AT cartridge (whose model number seems to vary), which goes for 10 - 15 bucks (used to be a lot less) has a conical stylus, so won't have the upper frequency analysis of an eliptical like the M 91ed, but is cheap and cheerful, a pleasent cartridge to listen to music with, seems to me the baseline. If you want better sound, its easy to achieve, and doesn't need to cost much, but its also possible to spend a lot and not get too much better. My recommendation would be to by it, or something like the cheap Ed Sauders Big Red (14.95) and get used to records, learn what you like, and only then spend much more.
There are generic stylii for the M 91 ed, and they are okay, in my experience, and that would be a different approach to spending little until you know what you want.
After that, a Jico Stylus might be just the ticket, or a Grado or an Ortofon, or whatever.