Well, you asked....

Ditch the 18cc motor or use it to make a gas powered "Margarita Machine". (blender)

The Homelite 25 or 30cc is probably what you will want to play with. There is a ton of info and lot's of people running them in boats, and you can find a lot of info, parts, and support. They will have much more power potential for about the same weight as your 18cc. Just do an internet search and you will come up with several websites on Homelites in boats. If you are wanting to run "Gas/Glow", there is lots of info here in this thread, or just ask if you have a question that hasn't been covered.

Here is one website to get you started:
http://www.warehousehobbies.com/index1.html

Scroll down and on the left hand side you will see "accessories." Click on that and the page that comes up has a link to Homelite parts and accessories for boats. (small print blue letters near the top of the page.)

Good luck,
AV8TOR