I think there are some articles on that here. If you can machine parts then converting a trimmer to rc boat use is easy. If you don't have the ability to make mounts you have a problem. I done conversions on a couple different trimmer engines,some were water cooled some left air cooled.
If you can make the parts,all it costs is time and if like me, I like making parts, then go for it and don't let anyone talk you out of it.I took it to extremes though, I scratch built a boat using the epoxy glass over foam technique, made all the hardware. The only thing I bought was a prop and radio gear outside of screws nuts and bolts.The engine was a freebie from a relative.