Yeah frustrating when bike doesn’t work. Bad battery can’t cause any damage, it just won’t have enough juice to start bike. Desperate quick-fix measures used by many mechanics
can cause damage though. Such as jump-starting bike from running auto. That will fry RR - regulator/rectifyer.
This might be issue with your battery not starting. Again, need to test and measure with multimeter to know for sure. Most mechanics are useless when it comes to electronics, which is why I do all my own work.
Here’s some bodywork:
https://www.ebay.com/itm/162277926358