With out actually seeing the machine and cabinet in question I dare say not difficult, at most time consuming. There are probably schematics for full rewiring available on the web, the vintage yahoo group files, or you can draw your own from the original. The cabinets I have seen either lets the cable for foot / knee control unplug to a separate unit, or it's wired directly to the motor, but usually with a plug. The older cabinets (pre 1940s - maybe a bit later) have a porcelain speed control part in the cabinets, knee control only; the later ones usually have a socket-plug setup. Replacement bobbn cases and bobbin are available.
Last edited by Mickey2; 03-22-2017 at 02:36 PM.