This one is a tough one. While it definitely has Mustang looks, the horizontal stabilizer is too far back to be a non-modified P-51 and the leading edge of the wing is too straight to be a Mustang. Here's a good shot that shows the differences: http://www.zap16.com/images/vlk04%20p51%20damn%20yankee%2002.jpg
http://www.508pir.org/images/spotters/UK/UK-Mustang.jpg
http://en.wikipedia.org/wiki/De_Havilland_Canada_DHC-1_Chipmunk