The wooden toerails should be straightforward for anyone competent with woodwork. You can scarf in the new piece pretty easily. There is little curvature in that area as well, so it should be a good "intro to woodwork" type project.
Epoxy will not serve your purpose here. It may work to stop rot from spreading, but it will not provide enough strength to bear to load of your track. Just go with the repair; it won't be as bad as you think.
I don't think the LWL will increase enough when heeled to hit 7.5 upwind, though those speeds are certainly attainable downwind as you come down waves.
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