I would mount the exhaust before trying to fire it ,so you don't burn or warp the valves , besides the motor needs some back pressure to run right. If you have fuel and spark ,what's left? compression,you might try spraying some fogging oil in the cylinder to seal the rings and prevent damage to the rings and cylinder walls until you have oil pressure ,and spraying a shot of starter fluid in the carb throat. when you set your valves did you rotate the motor ccw ,the same way the tires rotate? did you set the decompression cable ?You could try a new sparkplug,as some can be bad right out of the box.Tell us how you figured out you had spark and fuel.