Nawor3565

Master degree in EE here. It will almost certainly be fine to swap those batteries like that, I would be more cautious if it was some cheap Temu device but for a vintage Yamaha you’ll be fine.

One thing I will caution you about though, the synth almost certainly lacks under-voltage protection and could drain your 18650s below their minimum voltage. Make sure your cells have built-in protection or add some of your own.

Here’s what I would do: open it up, see if the ultrasonic transducer has a part number written on it. If it does, Google it and see if you can easily buy a replacement for a reasonable amount. If it’s not too expensive, I’d say it’s worth a shot.

Otherwise, yeah it probably isn’t worth putting any more effort into fixing.