Just going to 24v won't double your speed. You can get 24v at about 67 # of thrust and that won't be enough. I seriously doubt if the biggest one can get you 6 mph. However, you will get better performance, more staying power, and more speed when you go larger. If your budget allows, go for the most thrust you can get and add some length to the shaft of the new one.
If your tmotor battery has age on it then get 2 new ones. Get at least 27 series and as many reserve amps as possible (200 is ideal).