You may just want to check with yourself that you really want to do this. It is a pain to get back in again, but it is possible.
To remove the valve gear you have to remove the idler which can be seen showing 3 teeth in the photo. Its axle needs to be pushed out. A fine pointed screwdriver pushed against the pin will force it out of the chassis - it offers moderate resistance. Once the pin is removed, the outer plastic case of the gearbox can be lifted from the chassis. That comes next.