Hi Mabuchi540. Please dont shoot me If m not quite right on this, I dont own a fox, but did build a few many years ago.
The Tamiya Fox did have the option to change gear ratios, and the pinion sizes were from about 13 to 17 teeth. The pinions are 32dp, meaning that they are much more coarse than the modern tamiya stuff. If you contact your LHS they may be able to get them either from Tamiya or after market such as RW racing.
The manual should have the gear ratios listed in the back, if you dont have one you could try eBay, or looking at tamiyaclub.com.