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I assume you fitted the rings and located the pins in the piston ring groves to locate the ends of the rings, if you didn't know about the pins then you will have probably broken the rings. Also the piston has a marking on the crown to indicate which side faces the front of the motor that is very important to be done right. Try a few drops of straight two stroke oil down the spark plug hole and kick it over prior to refitting the spark plug, that will give excellent lubrication and help compression.
If it still won't fire up you will need to strip down the top end to find the fault.
A tip is to fit the piston into the cylinder leaving the bottom of the piston down far enough to fit the piston pin into the con-rod then fit the final circlip after setting the flywheel to top of the stroke and stuffing plenty of rag to hold the con rod in place and to stop the cir-clip falling into the crankcase if it isn't located. Be sure to fit a new cylinder base gasket and torque the cylinder head diagonally and not "around the clock face".
Hope that helps you ...