Hi Andrew,
no problem! 
Yes, the orientation wothout known coordinates is a bit wanting.
What I would probably try is to scale it roughly, then add two GCPs that you know are on one axis of your intended orientation, note those coordinates and make a simple transformation (as in calculate the distance and then set one of them to 0 (only for two coordinates, leave the third unchanged) and the other on one axis with the distance as 2nd coordinate. If neccessary, you might be able to repeat this step for the third coordinate.
But it’s all about the old project, if that doesn’t exist anymore, then it’s a moot point. However, you could just recreate it since you seem to have the images still…