I'm pretty sure this question has come up before.
Ignoring the legal and ethical considerations, and jsut dealing with the technical ones:

When you lay your glass on the outside of your original boat/mold the layup ends up being bigger than the original. If you use a layup of two layers of glass cloth, one of mat and two more of glass cloth you'll end up with a hull that is as about as thick as any commercial fiberglas boat -- probably somewhere between 1/8th inch and 3/16ths.

So your copy is going to have a beam that is a bit bigger than your original.

While the glass is curing on the original hull it will retain that shape. Marking it in numerous places will be critical, as once you remove the fiberglass shell you have created from the hull it was molded on, it will be very flexible and keeping it in alignment will be very difficult.

It would be good to construct a strongback to support the shell, but putting it on the strongback so it is straight requires you to build supports for the inside of the structure. If you build with the bottom of the hull up, this means getting under the strongback and working over your head while you try align the shell. It can be done -- with some difficulty. Copying offsets or making duplicates of the curves of the original might be useful.

A more common method is to use the original boat to make a mold, and then line the mold with your fiberglass. This way the mold keeps the hull shell in alignment as reinforcement is added to the interior of the hull to keep it straight, stiff and solid. Once the hull is reinforced it is removed from the mold.

People who usually deal with fiberglass fabrication would want ot make this mold from fiberglass. A spray gun that mixes resin with chopped stands of fiberglass can be used to spray on a thick coating. Additional layers of sprayed on fiberglas and reinforcements (blocks of wood, for example) can be embedded in this to give a very rigid mold. Once the mold is removed, you can finish the surface of it and then spray in more fiberglass to create the hull.
Sprayed on fiberglass is not as strong (pound for pound) as fiberglass made from woven glass fibers, but if you make the new boat thick enough and heavy enough it will certainly be strong enough -- and sprayed on fiberglass is fast to apply. Don't get me wrong on the quality of sprayed on fiberglass. Body parts for Chevy Corvettes have been made from this material for years.

If you don't want to use fiberglas for your mold, a simple, but heavy, mold can be made from a 2 inch layer of sand mix concrete plastered on the ouside of the original boat. This is further reinforced by pressing in sections of chickenwire or similar woven steel materials while the concrete is wet. After a few days you can add another layer of coarser concrete to this to provide more strength and stiffness, and to provide a squared-off footing to rest the mold on when the thing is flipped over. If you buy a lot of concrete at one time this may be relatively cheap, but very heavy!

I've considered other materials for making molds including plaster of Paris (as heavy as concrete and much more expensive but it hardens faster), and papier mache. The papier mache is easily made from shredding old newspapers (usually free) and a wheat paste which can be home made or you can get wallpaper paste. the problems with papier mache is that you have to put on thin layers so it dries well, and to build up a strong mold you need to put on many layers over many days. Again, putting in some sections of chicken wire can strengthen this.

All of these processes are time consuming and material consuming. If you are going to make just one boat and use a fiberglass mold made from glass cloth and mat, your cost for materials for mold and hull will be about triple the price of just the hull. That is because the mold must be made stronger, and so it uses more materials. You only get a cost savings if you are going to make several boats. This might be of interest if you wanted to start a boat rental firm at some popular marina and needed an inventory of cheap boats.

For all the time involved you could build a stitch and glue boat from lauan plywood that was lighter, cheaper and prettier.

Hope this helps.

PGJ