Choice of sprocket

Hello, This is my first post in the forum so please be gentle. I have had a 50cc china girl for about a year now and i am looking to make some changes. I currently am running a 10T drive gear and a 40T rear gear. The same setup is on my girlfriends bike but hers is much more peppy. The difference i believe is the tire size. Hers is running 26" wheels while I am running 28" or 700c tires. Here comes the math part of my idea. If i remember correctly the reduction within the engine is 50:1. At 1000ERPM(Engine) the output shaft is then going 20RPM (1000/50), making that drive 200 (20*10)teeth per minute. That then drives the rear sprocket 50TRPM(200/40) so.... by the circumference of the wheel (28*3.14 = 87.92") in 1000 engine revolutions I travel 366.3' or 4.165MPH(366.3*60/5280).

If I am achieving a max of around 25MPH or 6000RPM. Do i go ahead and place a larger gear on the drive or a larger gear on the rear to try for some more speed. I don't want to over rev the engine and really don't know what it is capable of. Thanks

let me start off by clarifying some of my errors.
1. The gear reduction is 1:4 not 1:50 ( I verified rather than assumed)
2. I made an error which caused my estimates to be off by a factor of 10.

I have found through more maths that what I am actually looking at with my current setup is roughly 5.2 MPH/1000 Engine RPM.

I am fairly certain that if I can find what the max RPM is for the engine or some idea of its powerband is that I will be able to figure it out.

Thank you for your advise.