I had a Solcat 18 a number of years ago.
It was a great boat, though a bit wetter than other cats when sailing fast.
Anyway, from my recollections, I'd suggest you might be able to actually determine how long the rivet should be by opening one of the deck hatches.
Through that opening, I think you can either see (directly or using a mirror) or at least feel where those rivets come through on the inside of the hull.
The other though I have is it may not matter that much. That is if the rivets are longer than necessary. (Though shorter than necessary would be a problem.)
What I'm saying is if the rivet is too short the rivet won't fasten correctly. However if too long, the head of the rivet will simply retract further into the rivet body, then break off as it usually does. The extra inside the hull doesn't affect anything that much, one way or the other.
One other issue you may wish to consider is how soft your hulls might be, due to the age of the boat.
My boat's 1977 or '78 hulls were becoming quite soft, before it broke completely in half...giving its all to me having a "way good time" sailing it hard and fast.
Just before that happened, when I replaced tramp track rivets I ensured they were backed with aluminum washers inserted on the rivet, inside the hull before I tightened the rivet and broke off the head.
I felt that those aluminum "backing" washers spread the load better and prevented the rivet from simply pulling through the soft hull, when under stress....though I really can't say. Maybe they would have held anyway...
It will be interesting to hear what others may have to say.
