Re: No stringers!!
I am rebuilding an 1985 Riviera bowrider, and my boat had no stringers either... well I guess it has one, but I call it the keel, (runs down the middle). It's really only used to support the deck in the center of the span going across from the two sides of the boat.
I feel that if it didnt have them originally, it doesnt need them now, unless you are planning on to over power it or something like that. It will just add extra weight to the boat.
Just be sure that it truly did not have any at all. Make sure there are no old spots where maybe they just rotted away, and all thats left is a trail where the were laid.
I am sure there will be other opinions, but I typically try not to change too many things from the original design (other than sealing allllll the wood up with resin or epoxy!)
Good luck!