Hi Ben,
Thanks for the response. You are correct about how to remove the control dial knob. I did that, sprayed some WD-40 inside, and tried to turn the knob again the next day but to no avail. This poor machine was left to the elements outside for who-knows-how long (albeit in the lovely mild Mediterranean climate of the SF Bay Area), but the guy who owned it took the back cover off the control panel and the main cabinet so it was truly exposed to the elements.
Things look fairly clean underneath, except leveling legs rusted and long gone, motor coils still shiny copper, all the hoses are shot, but I can easily cobble together appropriate hose replacements. And the ungrounded electrical cord is really scary, but easy to replace!