Holley 1904 woes

[Dan&Becky'sScout80]

1963 scout 80 with a stock 152.
I am posting this question on all three forums I go to, so if you see a “re-run” elsewhere, please bear with me. I rebuilt the holley 1904 a couple of years ago, to get my 152 running before I pulled it out to refurbish the scout. I figured the carb. would be ok sitting wrapped tight and boxed up on the shelf, but it was full, and I mean literally full of white powder when I pulled the float bowl. I ran gas with sta-bil through it before storage, it did no good, I must not have drained it well either. The carb looks like it is toast, so off to the junkyards I go to find a rebuild candidate.
My questions,

What the heck was all that white powder? I have seen gum and varnish problems, but nothing like this stuff. (I wish I would have taken some pics).

I know there are no scouts in any yards in the area, would a 1904 from say a ford or amc work on my scout? I know that they put 1904s on a whole lot of vehicles over many years. What makes one unique to the scout? The throttle body? The jet size? What would I have to do to make one work on the scout?

I have an old holley 1920 sitting around, but the bolt holes were spaced too far apart when I tried it. Would another model holley or other brand carb. single barrel work on my engine?

Any ideas would be greatly appreciated, I am getting so close to getting my scout back on the road.
Dan