interactive ultrasound pen -parts replacement search

[kokodin]

hello
I am currently trying to fix what highschool students have broken, ebeam system 3 interactive pen.

System works on a basis of emiting ultrasound bursts of noise and measure how loud they are by 2 microphones  determining the difference in volume  and how long the detected soun is as coordinate system while emiting infrared information which button is pressed.

Unfortunetaly students have decided to "take apart" one and only pen we have for ebeam system. We have imitation systems and we had spare parts pens for those , this is how i fixted this one 10 years ago. But this time i run out of spares and parts, and i can't even find anything similar. If anyone have an idea what it can be replaced with i gladly test your idea.

Missing/broken part is a ring made out of plastic film covered with some metalisation , working as piezo emiter of some sort.
Shape is kind of important because it needs to evenly transmit sound in all directions  , otherwise it would only work at far end of the screen and loose all accuracy close to reciever or one of the microphones loosing line of site to the other one. I determined pen itself  is electronically ok by replacing broken emiter with inards of arduino ultrasound proximity detector, but the small piezo emiter in shape of a metal disc work only in one direction and can't be installed inline of the pen, because tip piece go trough the center .

to make things worse company who made it halted production in 2019 and so did all the competetion, so getting spare parts become even harder

if there was some piezo strip i could buty and just wrap it around the emiter post it probably would work but i have nothing.