I've been reading your posts with real interest. I know that Singer provided a brochure for the 319 that shows how it could be treadled, so I would have thought that a servo motor installation wouldn't be the most difficult thing.
You may want to look at a FB group run by a fellow who installed a direct drive motor to his 319. I don't recall if the motor came from a 15-91 or a 201.
Given the history of the 319, that is that it was based on an industrial design, I suspect a servo motor wouldn't be beyond the capabilities of a 319.