RE: bigger engine
MSUPERVEE.... dont know about how a .21 would be in your boat - maybe someone will chime in who has tried that conversion.... MISSHYDRO put a .21 in a Proboat designed for a .15 and to use her description, "I cant run over 1/2 throttle, at 3/4 she'll blown off the water" - paraphrase actually but that was the result (and she is happy w/that).
As for a pipe, the .18 is kind of in limbo: CMDI offers a Mac 2.5 pipe which was intended for .12's primarily (though I have one one a .15) and then they offer a 3.5 pipe designed for .21's - your in the middle. Again, maybe someone can chime in who has tried both pipes on a .18 - or you can spend the $$$ and try both for yourself.