A50 uses no head gasket(cyl. and head cast as one piece), just cyl. to crankcase gskt. and uses long bushings for sealing the crankshaft( no seals used). It requires using 1/2 pt. of oil( do not use 30or 40 wt. non detergent motor oil that was the original recommendation) to 1 gallon of gas.Use a modern TCW III oil of your choice. The old style oils caused carbon to form and caused many problems. The higher concentration of oil acts to seal the crank ends and to seal the crank shaft rotary valve. A modern spark plug to match the original heat range is the Champion D9 #509 gaped at.030 Good luck and enjoy running your motor. When starting motor make sure to open the exhaust relief port.