Start with a thorough looksie to check everything is setup as per book, strip and reassemble the carbs and go from there, delving into the guts of an engine cos you have no power is a bit reactionary IMHO.
As far as the lifters go, YES, you need to replace them, and on first starting you need to follow the bedding in procedure TO THE LETTER, otherwise you will scrap the cam and lifters within a few thousand miles (if not less).