do a compression test. you want to see 120 lbs or higher. make sure the carb is spotless along with the petcock and that there is a good constant fuel flow. also check the exhaust to make sure its clean and clear. if all these things check out ok and everything (intake manifold, cylinder head bolts, exhaust bolts) is tight, then it should run. while you have the exhaust off, roll the engine over by hand and look up thru the exhaust port at the piston sidewall. if it is scored or has smear marks, the top end needs to be rebuilt.
oh, make sure your oil lines are hooked up and oil is flowing. especially the line that hooks up to the intake.