Mike, thank you for your advice. I've wondered about a VFD and 3-phase motor, and I'll give that some additional thought. That would certainly provide variable speed, which a jackshaft cannot.
Almost all of my woodworking machinery is old iron, and I've enjoyed the metal fabrication/design elements involved in the restoration of each of the machines. I don't think that welding or leg support mods should be required. Craftsman, Delta, and other manufacturers offered jackshaft assemblies, and lots are still out there. Not quite plug and play, but close enough.

Something like these period-appropriate examples would be ideal, but I don't mind modifying something or starting from scratch if need be.
I suspect that what I should *really* be doing is buying a new, or at least newer, lathe. This 9 x 30 lathe is sufficiently archaic to be holding even me back. Obviously little swing, and it has never known a ball bearing (though bronze/oilite bushings don't get enough respect). I think that most of my old machinery is actually superior to what can be affordably purchased now, but that is not the case with this lathe. So, I've also been considering a mid-grade lathe from Jet.
I will continue considering a VFD or new lathe, but in the meantime I remain interested in finding an old jackshaft assembly.