You're going to love that boat! I'm familiar with the 312's but not the 313's... but thanks to Han's extensive website, I browsed the gallery to get an idea of what you're dealing with. There are options for replacing your cabin door and if you take a look in the gallery you'll see various ideas. I'm assuming you want to get back to a stock looking polycarbonate (plexiglass) door assembly -- one hinged upper and two swinging lowers. Although it's intimidating, you'll be surprised how easy it is to fabricate with polycarbonate plastic. It's expensive but literally bullet proof and takes the UV well. Take a look on YouTube for "polycarbonate forming". The toughest thing here is to bend the upper. Cutting and drilling are done with common tools. By building a homemade strip heater you can make amazing stuff. You'll want to build a form out of wood that creates the profile of the bend radius of the upper door. You can fool around with building a prototype out of aluminum flashing used in home construction. Heat the wood form with a heat gun without causing it to change color. Using a hardwood (oak, maple, etc. - NOT pine) will work best. Heat the polycarbonate with your strip heater. Then lay the poly on the form and let them cool. The key is putting just enough heat to get it to move but not so much that it distorts. Again... a little web searching will provide you with many details. Polycarbonate is also easy to glue too so you can glue on a thin piece to lap over the joint where the two hinged door meet.
Well, I hope that helps get you going. Best wishes on your project!