To gain more power out of any engine there are a few simple rules.
1) "There is no replacement for displacement"
2) Increase compression ratio, which allows you to take better advantage of the fuel burn. I once read there is actually less energy in higher octane fuel. It is just refined better to eliminate pre-ignition in high compression engines. One advantage to low compression engines is they can run regular unleaded auto gas. 87 Octane for me, thank you very much.
3) Think of your engine in terms of an air pump. The easier it is to get air in and out of an engine the better power it will make. (Example, replacing the stock air box on my 1.8L Miata with a mass flow air intake and K&N air filter gained just under 5% more power at the rear wheels, dyno proven.)
4) The "stock" Van's exhaust seems fairly good, probably better than what the O-235 originally lived with, so you probably won't see many gains there.
5) Electronic ignition would be good. Better spark timing and a hotter spark will help the flame propagate much better and will reduce the problem of plug fouling from 100LL and produce more power.
6) Fuel injection will help. Carbs are ok, but fuel injection will be better. You have two real options here,
Ellison and the
Aero-Carb. Both require a fuel return line, which will be no big deal. I seriously looked at the Aero-Carb and was all set to use one until I found the correct carb for my O-290-D2.
7) Forced induction! Very cool but building a turbo or super charger setup might be beyond what you want to do.
Now for the important question; why do you want more power?
If the answer is to go as fast as an O-320 powered plane, get the O-320. If the answer is because you have an O-235 that you would like to use and you want a little more power, do the things above, keep the plane light and clean, and you will have a great flier.