If it is a small area that has rot, it is certainly worth repairing. If the rot covers large areas of the deck, then either factor in big bucks for a professional repair, or walk away. When I bought my 1975 Ontario 32 five years ago, there was a small area of the foredeck, beside the anchor locker, which had rotted core. This was because a previous owner had replaced the hinge fittings without properly sealing either the new or the old holes. The rest of the deck was pristine, without even any stress cracks. All of the original fittings put on by the builder had been sealed with butyl tape and the core around them was still bone dry. I know, because I pulled every fitting.

To make matters more interesting, the rotted core was plywood because this was a high-stress area. I cut open the deck, dug out all the old plywood, then replaced it with marine-grade okume plywood and laid down several layers of roving. I then learned how to properly gelcoat non-skid over top of the new glass.

Although it was very intimidating to contemplate the work, once I started doing it things became very straight-forward. There are a number of good Youtube videos on the subject to help guide you through planning and execution. Oh yes, I rebedded all of my hardware with butyl tape, as has been my habit for the last forty years of boat maintenance. That stuff has never leaked on me.