Actually, the Farsnworth item is a set of plastic housings with a reed switch in one and a magnet in another. They're easy to find if I want to take one apart to get the reed switch, and live in the middle of nowhere, but there's no way this assembly is going to substitute for a slide switch...
It's possible to fabricate one myself in the same space as the old one. I just don't have the right combination of ambition, patience, and expensive tooling.
I have seen very-small SPDT reed switches that would fit if you took a standard slide switch apart, machined the insides out of the...
If you really want to make your own, here's some good general info (maybe a little too much, actually...)
http://www.geofex.com/Article_Folders/xformer_des/xformer.htm
I am looking for a part (a switch) that I think ought to exist, but may not. I've looked at literally hundreds of websites, and a dozen paper catalogs, and I've found a couple of products (microphones) that use something like it, but no switch for sale.
The switch I want looks like a standard...