replacing an old yamaha tweeter?

[drewloveland]

Hey, everyone, I am VERY new at all things speakers/amplification, and I need some help. I have a couple of (very old) Yamaha speakers that I run through a (very old) Yamaha mixer/amp, all of which were passed down from my father-in-law...I use these components to monitor my recordings I do on my multitrack recorder and to play piano through (I have a digital piano.) Recently I noticed a very bad vibration/buzz coming from the speakers. I took off the front covers and listened individually to the large round speaker in the middle (the woofer? It measures approx. 8 inches diameter) and the small rectangular shaped thing above it (the tweeter?). The distortion (especially on higher piano notes) was definitely coming from the tweeters. I disconnected the wires and took these out of the cabinets. Now everything sounds smooth, but I'm assuming (and I think I can hear) that I'm missing something on the high end because I'm only running the sound through the woofers. I was hoping I could replace just the tweeters but have no idea what with because I don't think I could find these anywhere being as they are ancient. The speaker model number is Yamaha S0110T, and on the bottom metal underside of the tweeter it says Yamaha JA0556, 5W, 8 ohms, and also M70, whatever that means. The speakers on the back of the cabinet say 50W rms. What if anything would be a suitable replacement for these tweeters? I know these are probably not the nicest speakers in the world but the piano sounds pretty decent through them, and I'd like to avoid a large expense until I can revamp the entire system entirely. The dimensions of the tweeters are:
-the bottom cylindrical part, 1 inch or less
-the top where it flares out, about 2 x 4 inches

and they look a little like this from the side:
__..........__
\ /
\_____/
| |__| |

any advice would be appreciated
thanks