Hello Bill, Welcome to the forum glad you joined us
Yes you can still pedal your bike like a regular bike with the engine kit. But, their will be more resistance which will make pedaling a little harder.
Speed all depends on your weight and terrain and what sprocket you have. Most kits will come with a 44t sprocket that should get you around 25-30mph. You may want to look into...Welcome to Sick Bike Parts
. It's a jack shaft kit for your kit that will allow you to use the gears on your bike.