What directions are you following ? Do you have the spark plug in the engine ? It wont turn over very easy if it is in.
Installing the rear sprocket is the hardest part I think. At least it was for me when I first did a bike. You just have to take your time.
Do you have the coaster brake arm off ? The directions say to file your rear sprocket to fit but its easier to just trim the dust cap. More accurate too.
You said you cut the rubber "pieces" , you only need to cut one. One goes on the inside and one on the outside.
Here is more instructions with pictures :
Center Mount Gas Motorized Bicycle Plans Installation