Hi Paul. The shutter is missing the top half of the right air cylinder (as we're looking at it), which controls the timing. You're right in thinking it's probably not worth fixing. Another Wolly shutter like it might be easy to find, there are millions of them out there, and they are surprisingly serviceable - not accurate, but with repeatable timing of shots. The default speed on mine, when they are broken, is about 1/25 of a second though, not 1/100. Put some paper in the film holders and try shooting at what is now your "default" speed. You may just be able to get by with it until you can find another.
I don't know of any modern shutter that will take the lenses that are threaded into those old Wollensak's, though they may be around. It may be easier to pick up another lens for 5x7, depending on your need. You speak of portraits and of the equivalent lens to a 35 to 50 on a 35mm camera, but most people I know use a longer FL lens for portraits to avoid distortion inherent in the wide-angle, up-close shooting needed to get a head and shoulders shot that way. To each his own. An Ektar 203 f7.7 is a nice lens for the 5x7, if you don't need too much movement, and they come up pretty often and relatively cheap on the 'bay. A more "portrait oriented" choice might be something in the 10 to 12" range. I've gone the way of Packard shutters and barrel lenses to expand the "cheapskates lens kit" myself. You can pick up all sorts of interesting glass for next to nothing- as long as you don't need it in a shutter.