Back to the basics...any grit on the valve seats, proper tappet clearance, use soapy water to check for head gasket leaks. Turn engine backwards and listen at the ports for leakage through the valves. If your cylinder is worn out of round the new rings could be allowing too much compression blow by. If you can spin the engine with a drill to get it running the rings may seat enough to get it going with the pull start. Did you stagger the ring gaps during assembly?