I am not extremely familiar with the RPM 167 motor, but I do believe they do press a steel sleeve in for this mod. Am I correct? If so, it sure is an expensive way to ruin a perfectly good engine.
Obviously, if you are looking for a 167, you will not be racing this bike because it will not be legal. Assuming that is the case, you are better off going out and purchasing a 250. Just the power you will need, and a huge increase in reliability.
If you are racing, there are much better, perfectly legal ways to make that bike rip. In fact, for that much money, you can have not only a ripping-hot engine from Eric Gorr, but you can also have your suspension completely redone by a professional, to your weight and riding style. And suspension, by far, will allow you to go faster on a track than just engine mods.
By the way, I purchased my YZ 1.5 years ago for $1900. I cant imagine wasting close to that much to ruin a good 125.