"...can't build until I start to catch glimpses of the finished piece in my mind."
I find that happening also Acer. For me I believe need comes before inspiration in the sense that I, all my life, have needed to be doing or have an upcoming creative project to feel "right." I need to be thinking of the next thing, to see it in my minds eye, to cognate on it when I begin getting those glimpses. Actual inspiration might come from anywhere, often it blindsides me in a pleasant way, say perhaps two of those glimpses connect and a clarity comes rushing in. In the past when I was a working stiff coming up with a design or illustration was how I made my living and that meant there was pressure to create. Not so these days but I still need to be creating to feel like me.
At a base level it is simply that these old guns inspire me, challenge me, I like that, like having that in my life. The challenge is to make the next one better than the last one, to learn and to develop as a builder. The inspiration can be found, as always, lurking inside and all it needs is a spark such as seeing something outstanding, old or new, on this site or elsewhere to fire my imagination and get me sketching.
I currently have two wormy curly maple stocks and am thinking they would make fine early Virginia guns. I have a walnut blank that is long enough for a fowler... who knows, they might even become those if I'm not inspired to do something else with them.
dave