In an extreme case, if someone wanted to modify the E-100 scooter for 50% of stock speed (5 MPH instead of 10 MPH), it may be possible. There exists a 6T front sprocket for # 25 chain so stock 9 tooth down to a 6 tooth. For the rear sprocket, going from stock 47 tooth to 62 tooth would give someone a top speed of 5 MPH (in combination with the 6 tooth front). This may be useful for someone that perhaps needs to scooter to go up a significant incline that the stock scooter cannot negotiate like up a fairly steeply sloped driveway for example.
By the way, regarding the original solution of using a 55 tooth sprocket with the same 29mm center hole, one of the existing 3 holes lines up perfectly, at least on the sprocket I got. For testing purposes only, I rode the scooter around with only 1 sprocket bolt. I would think minimally, one opposing hole would have to be drilled, with 4 holes being "optimal" (for strength and reliability). I rode it around and didn't notice a lot of difference, except that it was a little slower, and required a little less push to get the throttle to engage the motor. I think it really need that smaller 7 tooth front sprocket as well. However, the center hole on that sprocket is too small to fit over the motor shaft so I may have to get it bored out, but then the teeth might be too close to the center hole so it might not work. I will take some measurements and report back soon. Today is Sat 2/3/2018.
It appears the 7 tooth sprocket with the 6mm center hole bore is too small to be bored out to 8mm for this E100 so I went with an 8 tooth sprocket which unfortunately also has to be bored out to 8mm to fit on the E100 motor shaft. It appears the chain will have enough room to engage the teeth properly, even if the sprocket is bored to 8mm. A set screw will have to be added to hold onto the flat part of the motor shaft.