To change gearing, we don't usually change the gears in the transmission box. We usually change the pinion and spur gear. I don't think you will need the alloy transmission case, unless you will put a ridiculous engine in the car (6hp plus)

There are a number of different gear ratios available, and a steel set is a good investment. I use robinson racing 18/55 pinion/spur. This increases the top speed of the car a lot. The default gears are 17/57. Check out this post for more info... http://www.rcuniverse.com/forum/fb.asp?m=7172298