The oil ratio is not your problem. You can run it at 16:1 and have it be razor sharp if everything is set up correctly.
Start with the basics. It's a 9 year old 125, so verify the high wear areas first:
- Piston
- Rings
- Reeds
- Crank
Once you are sure it's in proper mechanical shape then get on with the job of jetting it correctly for the fuel you are running and the conditions you'll be riding in.
It's a 125 which means if it's not internally tight and jetted sharp, it's going to be SLOW.