How can I get the most power out of my engine without removing the stock muffler?
Step 1 in making more power....
Remove stock muffler.
But you could remove the governor, or back out the throttle stop to rev to a higher RPM (which will make more power soley due to revving higher).
You could put a high flowing air filter on it, but not sure if it is worth it with the stock muffler. If the muffler is already the factor limiting the flow, I can’t imagine an aftermarket air filter doing much.
If it is the stock air filter that limits the flow (vs the muffler), then upgrading to an aftermarket air filter and rejetting the carb should give a little more power. But I wouldn't upgrade air filter without getting a less restrictive exhaust.
A camshaft with longer duration should help make more power. Since the muffler will limit the amount of instantaneous air flow, holding the valve open longer is the best way to get more air into the cylinder.
A smaller CC head will increase compression ratio. That should also help make more power without increasing flow rates.
Advanced ignition timing can create ponies without increasing air flow.