Clean it up, lightly sand the bore with #400 sandpaper wetted with solvent, clean it again, lightly oil it, and put it together properly with a new ring. That is normal, somewhat mild wear and simple carbon buildup and staining, and it will run great for a long, long time with a new ring, especially in an airboat. I have engines that looked like that in airplanes that turn over 9000 rpms.
I haven't had one that looked like that not respond well to a basic rebuild yet, in many, many years of working on them. I haven't had one lose compression nor blow up in many happy hours of flying, usually all day long per outing, and usually in 100+ degree heat. (unfortunately...)
AV8TOR
Last edited by av8tor1977; 07-12-2016 at 11:57 PM.