Small Engines (Lawn Mowers, etc.): make more power without a engine bore, valve size, head angle

Question
how can you make a small engine like a 3.5hp have more horse power without boring the engine?

Answer
There are numorous things you can do but typically it is cheaper just to buy a more powerful engine.

Additionally, the amount of work required to get more power typically exeeds the amount of power you gain.

You can shave the head, angle the valves, increase valve size, port work, carb....the list is long but parts can be very hard to find unless you have a junk yard out back.

What are you trying to do?
Eric