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.