If you don't have the manual with complete spec's, here it is:
http://www.prillaman.net/av123/82_av123-xseries-manual_v4pt3_web.pdf
You really need spec's. From the manual's spec's, you now have the crossover frequency of 1750Hz (@12db slope), 8 ohms and a sensitivity of 87db (@ 1W/1M) which you want to match as closely as possible. Those are the spec's you need before calling Madisound. You'd be so lucky to find the driver mfr and part# listed on the back of the driver and actually find a direct replacement! I've gotten lucky, but rarely. The guys at Madisound are the best in the biz and can advise you with the tweeter and any possible crossover parts. You may get lucky with 'plug and play'
The tweeter looks like a soft dome. Here's a close match:
https://www.madisoundspeakerstore.com/hi-vi-soft-dome-tweeters/hi-vi-q1r-1-fabric-dome-tweeter-116-mm-flange/
If you can't find one with the same sensitivity, they can tell you how to "pad" the tweeter down. Ask them about 'baffle-step' as well to compensate if needed. I would get 2 and replace the other tweeter as well.
Measure the face-plate diameter and give them a call. Good luck!