A number of after market aluminium motor mounts allowed larger pinions. There is also a wealth of tuning parts that were available back in the day, from japan and HK made by third party companies. Some of these allowed a wider spread of ratios as well as changing the pitch of the spur. The difficulty is to find them.
That is one of the problems tamiya has addressed by releasing the DF03 chassis.
Japan has a huge aftermarket industry but not much of it is seen from outside. There are entire shops full of parts over there.