Also a bigger engine is going to make the plane heavier. This plane already comes in pretty fast for a trainer as it is. A heavier plane is going to make for a higher wing loading, which will increase the landing speed above where it is already. In addition to that the landing gear mounts on these planes have been found to be pretty tender, adding weight to the plane is going to make those landing gear mounts more likely to break.

Normally things like these aren't an issue on a plane because modifications can be made, but I try to discourage beginners from making any modifications to a plane they are learning on. I do this because at this stage in their flight career most beginners just don't have enough knowledge to make the modifications properly. In the case of the P-51 PTS you would be talking about major modifications. I would stay with the engine the plane was made for.

Hope this helps

Ken