Welcome Ncdebval.
If the 99 is similar to your great grandmother's, then it is possible that the machine you remember could be a 66. Like Leon said the 99 is a 3/4 machine, From my understanding there was a 3/4 treadle, but they are few and far between. The 66 would be the full size machine and there are a lot of treadle cabinets for them. I do like the 99s, too.
From my understanding, there was a period of time that Singer had a motor that attached to the cabinet that could be controlled with the treadle pedal rather than a smaller foot controller. According to
https://en.wikipedia.org/wiki/Singer_Model_27_and_127 there was one patented by Philip H Diehl, but I couldn't find the patent for that one. I remember someone showing a picture of one that was on a straight leg treadle, but I can't find that one now, either
. I did find
http://patentimages.storage.googleap.../US1792818.pdf which seems to indicate that there was indeed a motor that could be attached to a treadle and use the treadle pedal to run the motor.
I did find
Neighbor gifted me with this today that is about an industrial cabinet that was made for either treadle or motor power.
Janey - Neat people never make the exciting discoveries I do.