Hi,
I bought a used Hobie 12 about 2 years ago, and have been learning as I go since then. I think you can easily satisfy all of your needs, as long as you don't expect it to be pretty.
Battens 3 big ones
My sail does not have battens, maybe because it is a heavy duty sail used for a sailing school. I would just go to Home Depot and find some thin wood and cut it to the shape needed.
Hull Drain Plug
I bought the following on amazon:
https://www.amazon.com/Aftermarket-Kaya ... BGCT2?th=1Tiller Extension
I am just using something that looks like a telescoping pole that may have been used for a selfie stick or camera that someone discarded. If this breaks, I will just replace it with some hollow steel rods I have, or a broom handle, or dowel, and drill a hole in it and then attach it to the tiller with a screw (this assumes that your tiller already has the slot for the extension). I find that about 25" is the perfect length. I can send a picture if you want.
Vang Block Lower
This seems to serve two purposes:
keeps the boom from rising
keeps hole on the boom from slipping out of the gooseneck.
If you have the vang strap on the lower mast, I would just tie a line very tight from the block or deadeye on the boom to the vang strap. If you don't have a vang strap, I would buy a hose clamp or something similar big enough to fit around the mast, and then connect something to that that you could tie the line to.
New Braided Line to match existing one
Can't you just go to West Marine and find a line with the same diameter, strength, and flexibility as what you have or need?
Before you go sailing, make sure you attach some line between something on the boat and centerboard and tiller so when you tip over, they won't go to the bottom and you never find them.
I very much like sailing my Hobie Holder 12. I have never sailed a Laser, but it looks to me like one is guaranteed to get wet in the Laser, whereas in the Hobie Holder 12 I will stay dry unless I capsize.