Unfortunately, 9/10 times if your deck is rotten so are your stringers, bulkheads, and transom. Holding up your weight isn't quite the same as taking on the weight of the hull, motor, gear, and 3-6 people on the water, anyway I'm thinking that core sample might unfortunately show you need a complete gut, and stringer/transom job as well. My boats transom (unbeknownst to me) was 100% rotten, and it still held up to rough use on the water at full weight capacity, I wouldn't have known how dangerous is was until I opened it up.
Fortunately you're on the most helpful and informative forum for boat restorations! Looking forward to pictures.