I bought this thing a couple years ago knowing the sonic head was not working right. That being said I am now trying to find parts and it's very hard without knowing who made this thing. The side of the drill head has model 500248 stamped in it but I can't find anything else on this thing.
You might try contacting Terra Sonic International, in Marietta, Ohio. They are a sonic drill manufacturer, and probably one of the "experts" on various types and brands of sonic drills. They have been building this type of drill for many years. Jeff