I think the main motor is loosing its full power. If you can get the 3 cell Lithium which is 11.1 volt (don't go over 1800 mAH), you will see more power. The spare main motor is not that expensive, so you should try a new one. Put the hat sink on both motors and they will last longer. Keep your flying session about 5 minutes and rest the motors until they cool down. Going brushless will be too expensive because you will have to go "separate" which means a nex RX, new ESC, new gyro.
As for the vibration, because you have crashed it so you have to check more than just a new main shaft. See if the spindle shaft (the one that goes through the main blade grips) is still true. Also, check the tracking while you are hovering and see if the flybar is level. You will want to get it run as smooth as possible for your flying success.