If a gimbal calibration doesn't fix it you might need to replace the yaw motor. Check that the yaw axis moves freely, and doesn't bind or scrape on anything when hanging level.
If nothing is bent or otherwise obviously damaged you'll likely need to have the yaw motor replaced. It can be done yourself, the motors are available online, but it can be a fiddly job. If you're not confident and the camera won't work in its current state it's best to send it away for repairs.
EDIT: I've just reread your post properly.... Where did you get the yaw motor from? Was it "brand new" off ebay or ordered through a proper distributor? A lot of places on ebay buy crashed birds and flog the motors and boards off in 'new' or 'like new replacement' conditions. Open the cover behind the roll motor, disconnect the yaw motor and make sure the contacts don't have corrosion or other damage on them...
