Please let me know if you have any questions.
I do not know what the biggest motor is that will fit in a Razor E300 scooter without cutting the frame. We list the dimensions of all of our motors on the pages they are sold on to help with determining if they will fit or not.
Unfortunately I have just about the same answer for the battery size question as I did for the motor question. Since batteries are rectangular boxes it is very easy to determine if they will fit or not though. We also have battery dimensions listed on the pages they are sold on.
Great to hear that the sprocket modifications and new lower gear ratio are allowing the motor to spin faster and provide more power. That is exactly what lowering the gear ratio is supposed to do.
Although I could not really answer any of your questions this time please do not hesitate to let me know if you have any further questions.