Would a 26t pinion fit with a stock 70t spur? 25t to 26t is an increase in diameter of 0.8mm so the pinion-spur distance is 0.4mm closer with the 26t. In my experience tamiya motor mounts provide a relatively loose mesh, certainly looser than I would set on my BD7.
I'm entering a stock spec class where only the pinion can be changed.