You're not using a motor. It's a mixture of pressurized air and water that you're using to propel the rocket.
I found making a pad a few years ago was easier than I expected. I got a length of 1/2" Sch40 PVC (I believe, it's whatever the size that just fits in the bottle) and heated it over a Bunsen burner on very low heat (candle would likely work too) until it had softened. Then I slightly pushed the piece in compression so that it bulged a bit. This proved to make a good seal with the bottle when placed on tightly. I put a 90 degree elbow in and attached the tubing to a piece of pipe. I took a cap for the pipe and drilled it out so I could fit a tire valve stem in, and then attached it to the end of the PVC. Then I took a piece of 2-1/2" PVC and mounted it around the 1/2" so that the rocket could sit in it. Drilled a few holes right above where the bottle's flange was and put a nail on a string. Total cost had to be less than $15.
At the competition though, they had a nice launcher that was purchased, with a nice release mechanism and a detachable seal for the bottle so you didn't have to tip the pad every time you mounted the rocket.
It doesn't matter what you use to pressurize the rocket, as long as you have an accurate gauge. You could put a gauge in line with your launcher if you don't have a bicycle pump or air compressor with a decent gauge.