Make sure, the propellers are mounted in correct order (2 clockwise, 2 counterclockwise - google for correct mounting), like shown here: https://ardupilot.org/copter/_images/motororder-quad-x-2d.png
If a motor is not working good anymore, you could find spare motors e.g. on eBay and Amazon. Replacement requires soldering.
If a motor is not working good anymore, you could find spare motors e.g. on eBay and Amazon. Replacement requires soldering.