Did you align the cam and crank timing marks?
Did a lifter fall out during install?
Is the valve lash set properly?
Does the flywheel have a key? Is it aligned properly?
Coil gapped properly?
Spark plug wire on?
Gas in the tank?
Gas in the bowl?
Switch turned on?
So remove the parts that make it run, add new parts, now it doesn't run. If it was me I would put the old parts back on and see if I could get it to run. It might be edu-ma-cational.