The top speed of the 2007 JOHN DEERe Gator 620i is 30 miles per hour. There are competitive models available that are said to reach up to 50 miles per hour.
You can not remove it, but you can change out the CDI box. http://www.magnumtuning.com/en/detail/revolution-pro-atv-speed-rev-limiter-cdi-box/john-deere/gator-xuv620efi/atv
Oh, the oil drain plug on a John Deere 620i Gator is usually located underneath the engine. You might need to look towards the back of the engine, near the bottom. Just take your time and follow the engine block down, and you'll find it. Remember, it's all part of the fun of taking care of your Gator!
There are actually two, one next to the oil fill tube and one on the opposite side of the motor, basically under the drivers seat. You will need a thin walled 13/16 deep socket to get on it, the one on the front side of the motor (under the seat) can be difficult to get on as there is a pipe in line with the plug.
you need a technical service manual to find that out, I don't remember off the top of my head, but it could be a bad sensor, the most common one to trigger is the air temperature sensor