Loose motor mount bolts are not always an indication of rotten stringers. I bought my boat back in 86, 4 of the factory motor mount bolts were already stripped out in the first season. I ended up installing the next size larger bolts. That got me by another 27 years. For the short fix I lag bolted and epoxed 2 buy 3 angle iron to the sides of the stringers that got me until last fall when the floor got a soft spot behind the drivers seat. That's where the ice chess sets most of the time This winter I did a cap off restoration. 5K is not enough.
Most likely time has taken its toll on your boat.