Well, Alley Cat offers a conversion for the PR Mk XI in various guises. Here is a nice build using it:
https://forum.largescaleplanes.com/index.php?/topic/48861-132-spitfire-pr-xi-conversion/
If doing it yourself, the unarmored one piece windscreen seems to be the most difficult. At such a large scale, the three O[sub]2[/sub] tanks will be visible:
Remove all traces of armament, including cannon bulges, and add camera ports with cameras visible. Wings are puttied smooth (in real life) to help with speed and there are new bulges under wing.
Photo Gallery of the Bowser Wing on Mk XIX which are identical:
http://spitfiresite.com/2011/07/guided-tour-of-the-spitfire-pr-mk-xix-the-bowser-wing.html
Photo Gallery of Cameras on Mk XIX which are identical:
http://spitfiresite.com/2011/07/guided-tour-of-the-spitfire-mk-xix-camera-equipment.html
I hope this helps,