I've had similar dilemmas with my 1:25 scale cranes that i've been scratchbuilding. With my first one i resorted to sourcing as many photos as i could, getting a small scale (HO/1:87 scale) version of what i wanted to build and then, using an easily identifiable piece i compared sizes (ie: knew the size of the tyres, so i measured the HO model, multiplied the dimension by 87, then divided it by 25 to get the size i wanted) this worked as i had already figured out the same dimension by taking the 1:1 size and dividing it by 25 so there for i had an accurate guage to go by.
Obviously, i had a lot of research ahead of me as i couldnt do this for the entire model but i went to the local stationary store bought a couple of decent greylead pencils, erasers set-squares/triangles and most importantly, hal a dozen sheets of A2 size graph paper (with the lines marked at 2mm intervals) Over the next few months i drew my 1:25 scale plans from the left, right, front and back and also two arial views, one of the superstructure and one of the carrier. On a seperate sheet i then drew the same angles but of the chassis, suspension, steering and drive train.
Unfortunately, a lot of the things we want to build, cant be done without this type of effort as a lot of the vehicles are either too old to have plans or to new to be 'handed out'.
