There is nothing inherently wrong with the PTS engine which is part of the P-51D Trainer package.

That said, the limiters can be problematic for someone not used to them, especially if they do not permit you to adjust the engine to it's optimum performance range.

Check the long PTS thread here.

There is a lot of info there which goes into improving the plane.

Don't be put off by all of this as usually all of the provided info is not needed, but it may be a good idea to peruse the rather long thread.