Edward i dont like the word removal but if he has to do so he will have 3 head gaskets to choose from. Gasoline only, dual fuel and lpg.
The timing gears are pretty much bullet proof on these units as i have never had to replace a set. Possible if the engine has been run hot hot that the valve seat has dropped. I am hoping he performs a comp check before pulling the head tho. A dropped valve seat will be easy to identify by checking the return height of valves with a straight edge laid across valve train with rocker rack removed.