Ninja are great engine so long as you don't over load them with fuel. a few guys at the track have been having a few problems with this. os speed great engine but is almost double the price of everything else. axe rossi is great if you are looking for a tighter track it does strugle in the mid rpm but good after that.
i am going to an alpha. all that i have heard about them is good and they have a great powred band that is contistant threwout the rpm range and they are very cheap.
Alpha is my bet