You could also look at windscreen motors off older cars which will work off 12v batteries, freinds I know use the 7.2Ah batteries that I use to power their boats … we recycle the batteries rejected by communications gear. Some [most] are AOK for model use.
If you want to go petrol then there are the weedeater motors [30cc and 22cc]which we use to power our quarter scale jetboats but you would need a gearbox to split the power into the twin props? Our boats when properly tuned go faster than older members can handle 