When you get the big bore kit, get it done through Forward Motion (Eric Gorr). He will do the big bore kit and then flow port your cylinder.
http://www.eric-gorr.com/
You can also go to a thinner head gasket. This will raise your compression. If you do this, you will be forced to run race fuel, because pump gas will not have enough octane. You will also want to custom cut your base gaskets. You have to make sure that your base gasket is not over lapping onto your transfer ports. Custom fitting the base gasket can give you an instant increase in horsepower.
You are at a disadvantage right now, but the extra 20cc's should make you even with the 250 4-strokes. A 250 4-stroke is putting out about 6hp more than you and the power is more linear.
The 125 2-stroke to the 250 4-stroke is the worst match up there is. A 250 2-stroke is more evenly matched to a 450 4-stroke.
Most 4-stroke racers are lazy. They will try to stay in one or two gears the entire race. They will also let the engine do the braking for them. The secret to beating a 4-stroke is to brake harder and deeper into the corner. Most lazy 4-stroke guys will always follow the berm around a corner. You need to be trying to square off the corners, by slamming into the berms and bouncing off of them. Use the explosive power of the 2-stroke to rocket you off the berm, instead of trying to ride around it. Granted you can't do this for every turn, but there should be at least 2 turns on a course that you can bounce off of instead of riding around.
Since you are racing a 125, you also have to keep your rev's up. Be ready at any moment to fan the clutch to bring the rev's back up if you end up in too high of a gear. I have raced and beat both 250 4-strokes and 450 4-strokes with a 1991 Suzuki RM 125 that has a FMF Gnarly pipe and Pro Circuit silencer. It is not easy to beat them, but it can be done.
Make sure you keep your power valve clean, change your crankcase oil after every week end, keep your shock linkage lubricated, keep good rubber on your rims, change your piston rings after every 12 races, change your piston after every 24 races and change to a clean air filter after every race.
