More than a hundred dollars for a motor? That seems alot for a motor but a two-speed Sanitaire would be fun to play with.
I personally would not 'upgrade' any vintage vacuum unless I knew there are many in collectors' hands or the original motor was unable to repaired. There are many Eurekas around, and if you're happy with it, then it's worth it. If you can, could you post a video of the new motor a low-speed?