Gizmo.
I don't think this engine will need any mods to improve power; it already develops around 7 bhp in standard form. If I think it needed it, I would do the mods myself as, a) I live in Australia and b), I am able to do it with access to machining and milling equipment and advice etc. I'm a veteran DIY type, with a wide range of experience.
Besides, Av8tor describes modifications of similar engines very well on other threads here on these forums.
Thanks anyway. It's good to know that the engine mod hobby and industry is so alive and active in your country.