Hi Simmo
I've not seen a WW1 a/c without a belt, but saying that I had a close up visit too the replica at the Shuttleworth collection,and it diddn't have one!! I'm building this kit at the moment and I'm going for a lap belt.
Have a look here,this may help with other parts of the plane
http://www.pjvision.nl/index.html
HTH
Andy