Around 20 years back I went the chainsaw motor route on a build. Spent a few days grinding off excess moldings from the original saw housing to reduce size and weight. An additional problem is chain saws rotate in the opposite direction to 99% of props and shafts on the market (though there are still options). While I ended up with a boat that did run (an Expresscraft apache as a matter of coincidence - Tom who posted earlier likely did the glass work) I also spent more time and effort making something that was plagued with problems every step of the way. It was one of the many valuable learning tools I have played with over the years - but not one I would encourage anyone to attempt unless they are very skilled with the ability to fabricate custom parts and hardware.
Chainsaws generally do not make more power than most RC motors and typically only rev to 7000 - 10 000rpm. Most Marine motors will run in the 16000 - 18000rpm range and out of the box will weigh a fraction of the weight of a saw motor. While the bigger displacement saw motors do pack plenty of torque - this too is something you most likely will not be able to tap into without an elaborate overdrive setup that only complicates things (and robs power in the process).
I would really encourage you to look into either a weedeater motor or a true RC motor (preferably water cooled). They have so much more potential, upgradability, resale value, and all the work has been done for you. I encourage you to take a look through some build pics I got from a craftsman in Germany which may give you some tips or inspiration for your build.
http://imageevent.com/justaddwata/mo...ipsfromgermany Keep us posted on how you proceed and do not hold back with the questions.