Has it had the oil injector pulled off to run premix? Stock they were oil injected, not mixed.
On the power, once you get the injectio/premix situation answered, start by going over the fuel system, compression, spark etc, and see what you find. An engine needs air/fuel mix in the correct proportion, adequate compression, a strong enough spark to ignite the mix, that spark needs to ocur at the correct moment, and the engine needs to be mechanically sound, with no air leaks or other mechanical troubles that will interfere with these. When these are not present or are not in correct amounts, you have issues with poor running.