Im about to buy this motor. I want to get a little more power out of it. What can I do?

I’ve got the 13 hp Honda gx 390 on a Dixie short tail. This is what I did.
PRN open air filter
Jetting carb 4 times and reading plug
De-baffled the exhaust
Indexed spark plug and gapped at 0.040”
Lapped valves (Cost $ 20)
Played with the valve adjustments
Advanced the timing (filed timing key rotated flywheel)
Ported and milled the head myself. (Used a mirror and sand paper) really it worked)
Installed thinner head gasket
Adjusted governor to the max
Added tiny tach to get the most in water RPM for my prop (This was the most important tool to maximize my motors output) It allowed me to tweak maximum power and performance out of this motor. Trial & error playing with the timing, compression, jetting, valve adjustments, etc
Current settings 24 mph at 3900 RPM with a 9x9 prop on an Alumacraft 1442. I think it’s at 20 hp at this point but not sure. I usually run it at about 20 mph.
Tons of race parts available for this motor. NR racing, Parsons racing, affordable go karts etc.

Lessons I learned on this build
1. Pay for the HP from the beginning.
2. No substitute for displacement. Go bigger
3. Under load is when the small HP motors bog down. The big boys always have power.

Good luck !