Mecury 5 HP 4 stroke

[The Viking]

Hello friends I am new to the forum and wanted to consult on a Mercury 5hp 4 stroke have been in the service manual cutting and the pieces from 5 hp and 6 hp sonmuy few, is there any upgrade kit and there is some visible edge get 20% more power, because when you buy the salesman told me it was going well as many auxiliary motor boat and try it seems to me that he fell short, and the maximum does not pass the 4500rpm pushing the boat, I did check for the official service and told me that everything is ok, I advise you to change the pitch of the propeller or exchange it for a 6 hp that comes with the same prop.
From already thank you very much.