All the gears both stock and heavy duty are metal. Which ones do you want? I run all the HPI heavy duty gears as they are lighter, stronger, machined much better. I all so run a titanium lay shaft which is stronger and lighter. I so not run a alloy transmission case as I am weight conchs and they are to heavy for my liking.
Does your 5b have a plastic or alloy diff housing not the transmission case but the diff housing. If it is plastic you really need to up grade to alloy. Where are you located? You can all so down load the manual online for free since it seems you do not have one.
Here is a picture of a stock gear and a heavy duty gear. the heavy duty is the thinner of the two.