Wood rot?

I just purchased my 1st boat. It is a little 16ft Vanguard early 80's fiberglass boat. Problem is, where the motor mounts onto the back of the boat is rotted inside...how do I fix this? What material do I use to replace what has rotted inside? I have a friend who does body work/fiberglass who can do the finishing for me no prob, but I am not sure what to use to replace what has rotted away inside that back end. What willl be strong enough to support that 70hp Merc outboard and do I have to worry about the weight of what I use?