no such thing as dry rot. there is rot that may have dried from being stored in an area where it could not continually get wet.
a 1992 four winns is an OMC boat. not the most stellar build quality back then.
only way to tell if you have moisture or not is a moisture meter.
welcome aboard
you will most likely have to either make the cushions yourself or pay a local upholstery shop to make them.
each pontoon boat maker usually makes their upholstery in-house because its cheaper than buying from an upholstery shop
keep in mind, idlers are the cheapest bit of aluminum casting with the lowest cost spring and the cheapest bearing known to man - the 6000 series with a shield (no seal) they are designed to last about 8-10 years in the automotive market
being an L29, your boat is between 22 and 27 years old...
remember, marine fabrics range in cost from the low end of $5 per yard to the upper end of $100 per yard. Sunbrela is a good material in the $15/yd range. upholstry is where you want to buy good fabric so you dont have to redo the job again.
generally marine interior fabric meets the same anti-microbial requirements of outdoor fabric. however doesn't have the UV stability of outdoor fabric.
sunbrella is one of the many marine upholstery companies
you have to judge for yourself, your boat, your skills, the geography of your surroundings, etc.
for example, a 40km/hr gust in one location may simply knock your hat off, in other locations it may cause 1.5-2 meter swells
the cost of new and reman is higher than the higher volume V6's and V8's which started life as truck engines as this engine is a GM industrial engine and is only used in trash pumps, fork lifts and entry level boats.
welcome aboard.
since you have a cottage, my recommendation is a pair of jet skis, a pontoon boat, keep the fishing boat, and also get a few kayaks.
to have a bunch of people in the boat to slowly tour the lake - pontoon boat
to go fishing - fishing boat
for water sports - jet ski
for quiet...
what is the shape of your tank? is it fairly shallow and under the floor? bow mounted? stern mounted?
what angle does your boat sit at while static? on-plane?
I know of many shallow, below deck tanks that dont read worth a carp because they are long, and shallow and the boat sits at enough...