Digger.
I don't understand Why you want more power??? If you plan to run in an OMRA race there is a 50.8 cc limit.
You say your Zen is tuned 26cc. I don't know what boat you are running but it should be doing somewhere around 40mph. There is some info on the OMRA site about the fastest speed OMRA boat get to. Remember that OMRA racing is NOT about out and out speed. It is about staying on the coures for the race duration. Often it is the slower boats which take the winning places.
Spark plugs, Prop size, even how much air getting into the boat when it's running all have to be right. How much and what oil do you use in the fuel? Do you have trim tabs? how are they set? How big is the fuel tank?
It all plays a part on how fast your boat will go.
If you post some pic's of your boat and engine set up we can have a look.
Cheers for now.