If you have no compression, it sounds like something is severely worn. Your cylinder could be worn out, may be time for a replate for nic, re-sleeve for regular cylinders. You will have to match the piston to the new bore. Did you install your rebuilt piston correctly? Arrow to the front? Rings in correctly, wrist pin circlips locking correctly? Did you torque the cylinder base and head to specs? Spark plug installed correctly? Lots of factors, but rebuilding a single cylinder is very easy with the manual, you have a shop manual right? Hmmkay...