How ironic..... I have 2 boys, ages 6 and 3 but soon to be 7 and 4, and my wife works a second shift as a nurse so I always have my boys in the evenings and have to either wait until the wife gets home late at night or I have to wait for weekends to do more complicated things. This year I have finally been able to do more being that the youngest is almost 4. I don't believe in daycare...... not even if it means I get to play with boats, motors and fish more. And that is saying something from me :lol:
As for the cylinder head...... There is not much that you can shave and I have read articles where the pistons end up smacking into the head so you may just want to leave that scenario alone. Especially if you are looking to have a reliable motor.
For the intake manifold..... you can port the intake a little by putting the gasket on it to see if there is any that can be removed allowing better air flow. I wouldn't polish the inside any better than say 150 or 120 grit sand paper, you want some turbulence to allow the fuel to break up or atomize when going through.
As for polishing the head.... I wouldn't mess with the intake part of the head because of the possibility of causing poor fuel atomization, but if you can get in there to polish the exhaust ports then I would do that.
I have always wanted to make a custom tuned exhaust for my 88' 30hp johnson. I did a modification to my 9.9 a few years ago which included putting the 15hp tuned exhaust on it. Then when I removed my 30hp powerhead I noticed it had a very similar square exhaust as my 9.9 had, which made me wonder how much I would gain if I had the equipment to make a round style tuned exhaust for my 30? Reportedly it added 1.5hp to the 9.9 mod I did.
Now there is also the ability to change the size of the jets in your carburetor, but this can be a tricky thing to get correct. And I haven't yet figured out where to get just the jets from. But this requires a lot of putting them in, testing and pulling them out until you get the desired performance. I have thought about actually putting a totally different and larger carburetor on my motor to see if I could get a few more HP but......... I run out of time and money to do these ideas usually lol. And the wife only has so much patience for my motor mayhem.
You can never take to many pictures, for your own reference and possibly others in the future. I have hundreds on my computer and try to remember to snap pics when I am tinkering on my motor so as to be able to explain to others better when I can.
21 mph most surely had something to do with low compression. You should be able to easily hit 25mph and possibly 30. And as for the prop, when you get your motor up and running and you get a tach on the motor we can get you dialed in on the best prop that will give you top performance. I am running a 14 pitch SS on mine and I get 28 to 31mph depending on how much weight I have in my 14' boat.
Ok well I rambled on enough for now...... sorry for the long message