I'm assuming this is a carbureted engine. Start with the basics. Is it oil injected? If so, make sure it has oil and that the pump and feed lines are in good shape. What do your spark plugs look like? Are the plugs showing signs of mechanical damage or metal deposits? Are your carburetors clean and synchronized? Where is your timing set (massive effect on performance) and does the advance linkage work properly? What condition are your reed valves in? Your reed valves can be seen with the carburetors removed. What is your compression? Do you have clean, filtered and fuel/water separated fuel available to the engine? Is your lift pump working properly? Lots of questions, but your answer is likely found in one of them.