small engine repair is fairly simple. i didn't know crap about it until i started racing motocross and having to rebuild bike engines. you basically just buy a kit and at worst...watch a youtube video about it lol.
if he sank it you'll have to basically need to remove the head and make sure there isn't any water on the pistons, remove the carb for cleaning/draining, drain and flush the oil.
as long as he didn't compress water and fuck up the piston on the one that sank it should be as simple as that. you will just need a few gaskets for when you put it back together.