Proppy is right, that really needs to be fixed from both sides, depending on the whereabouts of the crack in relation to the interior/motor it may not be as big of a deal as you think.
Several questions, but here are a few...
How well/bad does the trailer fit? you really do not want to go through the pain of repairing it do just have it happen again...
Is this the strake where the stringer is (or would be if it continued on a quarter string deal)?
any other signs of stress cracks in the immediate area? Would also hate to see you spend the time on it if it is structurally FUBAR...
Unless you are totally in love with this hull, it may be that time, you could change over your parts quicker than fixing the one you have, just food for thought.
Sorry for the beginning of summer sadness, that sucks.
Also, that is not rust you are seeing, that is a sign of rotting wood, the wood is decomposing and giving off its oils.
GT