I have found several lately by doing a Google image search. While 99% of the images are photos (they may be useful too), there ARE some scale drawings in many cases.
Don't worry about the scale of the drawing. With digital photo editors today you can easily resize/rescale a scale drawing.
Another option is your local library looking for books on WW1 aircraft.
A good bet is to search for books for two good illustrators who used to do scale drawing for model mags, William Wylam and Paul Matt. Many of their drawings were republished as books of collections of these drawings- there are several volumes of each. The most recent is two volumes of Paul Matt drawings. These are currently in print and do include some WW1 stuff, though only American planes.