If they tried to change cam gears while still in the engine, I could see them knocking the plug out and doing damage to lifters. It takes heat and or lots of pressure to install cam gears, if a hammer was used, you can bet something was damaged.

Pull the side covers and lifters and check them closely. The cam should not have enough free travel to hit the frost plug if the gear and retainer plate are on right. If they forgot the retainer plate, the front of the block or back of the gear should really worn.

Joe