To make more power, an engine needs to breath better. That may mean, but not limited to, bigger carbs, bigger intake runners, bigger or more valves, different camshaft profile. It may also mean changes on the exhaust side as well. Increasing power at the top end, often results in a weaker bottom end and midrange.
For years, some people thought a jet change was all that was needed to turn a 99 cubic in. 90hp into a 99 cubic in. 140hp. While it is doable, and a lot more than jets is involved, it is always cheaper just to get the higher HP engine in the first place