The problem with modifying an engine is knowing what to mod., how much to mod. it, and when to stop. I had an os tz .12 that I simply copied what I saw on other professionally modified motors. I took it one step at a time. I started with the head button mod where you cut grooves on the inside of the head to direct the fuel air mixture toward the glow plug. This gave a noticeable improvement in performance and had little affect on fuel mileage. The next mod. was the crank scoop. This had very little affect on performance and the fuel mileage was slightly down. The next mod was putting tear drops in the sleeve and this was the worst mod for this motor. It had no affect on performance and took fuel mileage from 5+ minutes a tank to 4.5 The motor was now useless because the mileage was terrible. But, the piston/sleeve are cheap so I replaced them.

The thing you have to know is that these results only apply to my motor. Since all motors are different, you have to know what certain mods will do to certain motors. It can be an expensive experiment when you don't know what you're doing (like me ) So i've learned to leave the modding to the people that can afford to do it. Plus they can test them and use actual measurements and CNC machines (some even have dynos) to do it right.

If you really want a modded motor, look at which modders win the big races at big tracks. See if their motors got good mileage or were the guys pitting early with a splash and go at the end while everyone else was making 5min. Do your homework since these motors are expensive to begin with and paying a modifier usually isn't cheap. Hope that helps.