That gear combo should physically mesh just fine. In fact, you should be able to go as low as a 12T pinion if it suits. And since you think the motor is overheating, it's gonna be necessary to change to a smaller pinion anyways.
PS: Are you still using the original Tamiya spur gear? If so, are you also using Tamiya pinions? Tamiya gears don't work with other brands of gears because they are not the same pitch. Tamiya uses metric gear pitch, while everybody else uses SAE (imperial). For standard TA05, you need 0.6 module gears (not 48 pitch). These are sold in Tamiya's line of "A.V." and "H.P." pinions. The smaller HP ones are made of steel, and the larger AV ones are aluminum...
If you ever upgraded to Tamiya's fine pitch gears, they are 0.4 module (not 64 pitch)...