Al, a couple of things come to mind. First is that even though the rings may look like plastic they likely are not. They are blackened so they will not look like the shinny aluminum machined parts (like the piston). Just for kicks I grabbed one and stuck a magnet on it so most likely it is a common steel. Try it for yourself perhaps. Second is that when you install the piston to the cylinder make sure the rings are aligned with the pins in the piston grooves. That is to say the gaps in the rings should meet so that the pin in the groove is between the ends of the ring. Otherwise the rings will hang up on the edge of the cylinder and break. I'm guessing you broke the top ring? You shouldn't need to tap the cylinder to get it on. Now that you have though, look closely at the mating surface on the cylinder where it will seal to the head. Make sure it didn't get dented when you were tapping on it or the head will not seal and you will not have compression no matter how tight you crank down on the nuts.
Hope you saved a few bucks on that motor. I bought mine on CL thinking I would and the problems I listed were the problems I found. Didn't save any money to say the least.