I have to agree with Mart on this one, that a nitro engine sounds like a better option to use over a strimmer/chainsaw.
They are compact in size and believe it or not just as powerful as a strimmer engine in a boat!
The advantage is there is no conversion needed of this type of engine as all parts to are available off the shelf at reasonable prices ( water cooled cylinder heads, motor mounts, flywheels and couplings.)
There cost would be about the same if you purchased 2nd hand as a strimmer engine would be or even new you can pick up a size .12 for around £60 up to sizw .15, .18, .21 & and the .28 size can cost between £100- up to £250 ( i say .12 bla bla bla sizes as that was the lingo in nitro sizes from my race car days not sure if that's changed over the past 10 years to engine size in capacity 23cc etc)
Going by old money
i would have though a size 12 or 15 would be more then enough to propel a boat along quite nicely.
Only bug is you will need to to pay out for glow plugs and its starter, tuning is temperamental but once sorted they run for years with little modification to that setting, biggest bug of them all is fuel cost, nitro mix i think last time i looked was £22 a gallon think VAT has pushed it up a bit more now so worth shopping around. Be sure you keep to the right nitro mix , 15% 20% 25% 30% as fuels vary depending on there use, my 10th scale rc car (touring) ran a size 21 on 20% nitro , where as my 1/8th buggy ran 30% in a 28 size engine.
( i ran higher nitro content on the buggy to get the most out of the engine in regard to hill climbing events, and rpm.)