Mod1 is the tooth size on the pinion. The pinion that came with your inferno gt originally was a clutch bell (not a pinion) and used the same Mod1 tooth size. Not sure how you would attach a clutch bell to an electric motor unless you came up with a custom motor mount for your inferno and use the Tekno clutch thing. The stock spur gears are Mod1 size and so is the metal spur you are using on the 1 speed setup, so yes you will want to buy a Mod1 pinion unless you want to try and figure out how to convert the gears on the car to something else. Doesnt make much sense to do that though. Much simpler to just get the proper pinion gear.
Yes, RC-Monster sells the Mod1 size pinon gears. There are other places to buy them as well, but we all know that getting them from RCM is the best way to go.

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