ORIGINAL: Rustytrax
Just adding to the above, I would say a must is the metal replacement elevation arm.
Yup... that is a given. It's plastic so it needs to go.
Also an inexpensive option is the gearbox mounting plate that Daryl Turner makes. This gets rid of the stand-off mounts and adds a little rigidity to the lower hull. Cut from aluminum... all my tanks and serious competitive tanks have them.
Also, if you have had any dealing with a clutch-type gear going bad (and gluing isn't your bag) look into replacing them NOW with the blue gears. This will save you from having to tear down your tank later on. Although you do loose a little safety precaution with the clutch (especially getting the barrel hung up in a branch) these won't fail on you later down the road.
You can go crazy with all sorts of items and issues, just depends on what you're going to do with the tank!
~ Jeff