I do my own modding and have modified alot of engines. Twostrokes are easy to modify for more power but the fourstrokes are not that easy to hop up. On my fourstrokes all I do is to drill out the outlet of the muffler some, and remove a headshim or two and reset the valve gaps. This does not give very much power increase, only maybe 100-300 rpm. More can actually be had by experimenting with different fuels. I do this mostly because I like to fiddle with my engines and I want them to perform their best

If you need more power then a given fourstroke can provide it is best to get a bigger one.

The Saito 100 is great as a hop up engine in the 90 size because it weighs only 600 grams and most other .91 fourstrokes weigh 640 grams