Firstly Welcome to PW! Like you I'm new here too! Like you I also started a project! Like you I have similar previous hands on experience on bikes! Like you I do too love CG125! but......unlike you I never do what you are planning to do on CG125. Still there is nothing impossible and everything can be done.
1. 5 SPEED GEAR BOX CONVERSION ON CG125:
Yes you can change the gearbox to 5 speed. It actually is a fairly simple mod. Most of Chinese 125cc copies of honda have 5 speed gearboxes and their gear set can be easily salvaged to convert your Cg125 into a 5 speed. If you wanna spend more Deluxe gearbox will also fit........this was the good part....... however there is a negative aspect to it too. Most people think that the 5th gear conversion will give them first 4 gears just like the original and there will another gear on top of it to increase top speed and fuel efficiency....and guessing your motives Im sure you too thought the same..........right ? right?........LOL ......please be noted that 5 speed gearbox of Chinese 125 or even deluxe have closer ratios than on 4 speed gearbox so after the conversion you we have to change the gears sooner and even in the fifth gear you will not feel a big difference. I have ridden a 5speed converted cg125 have its nothing special. So it would be a waste of money and energy for a very small gain....but hey! Its possible! and it will give a a little bit higher speeds on lesser RPMs......so its up to you!
2. Changing Carb to Suzuki 110:
Again Yes its possible and yes it will decrease the fuel efficiency. I have seen people running their cg125 on CD70 carbs..........they partially block the air intake to compensate with the lesser fuel entering the smaller carb. If you want to reduce fuel consumption why dont you try to adjust the original carb to lowest setting.......It will work. If the results aint satisfactory as per your standards you can choose to ditch the original carb with any smaller kind.
3.Reducing bore to 100 or 110cc:
Again Yes dude.......its possible....... you can insert a sleave into the cylinder to decrease the bore.....any good lathe wala will happily do it for you. But remember thats not the only modification needed. When you decrease the bore you also have to change the piston and weight (counter weight) to match your decrease bore size.
So yes! all what you said is possible........ also if you want to increase fuel effeciency you can try the following:
1. swap CG wheels and tyre with narrower cd70 wheels and tyres....
2. change the bigger fuel tank to cd 70 fuel tank as you wont be needing any.
3. replace CG brakes to cd 70 brakes to reduce weight.
4. replace cg seat to cd70 seat
5. replace CG final drive sprocket to cd70 sprocket.
6. replace CG silencer with CD70 silencer, it will actually increase fuel effiiency.
After doing all that you will have a bike very much similar to a CD70...........
I hope its not too subtle for you
AGAIN! Welcome to PW! We hope and wish you achieve whatever you want to achieve with your project!