RE: Custom rc boat STUCK!
Welcome to the forums.
A stock weedeater engine will push your boat just fine at somewhere between 10-15mph. No need to water cool it if you keep it open like you have. You are going to need to beef up the stringers a bit. Make sure you scuff up wood and glass them in.
Tell us what you have in mind for her and I am sure you will get lots of help here.