Hi there. There are a lot of Petie the 2nd repaints posted here. Most are for the payware A2A Simulations Mustang.
This is the only one that will fit the default Racer.
https://simviation.com/1/search?submit=1&keywords=Texture.Petie-2.zip&categoryId=&filename=YMake sure you paste the folder named Texture.Petie from the download into the P51_Racer parent folder.
If this is your first new variation to the P-51 renumber [fltsim.xx] to [fltsim.9] in the new aircraft.cfg entry.
Here it is in FSX-SE
Hope this helps.