where can I find a NACRA over rotator?
Can I use a hook and ring main hook system with my 5.7 over the top masthead?

Murrays has the rotator arms and blocks you would need. I would leave the hook system you already have for the halyard. It's pretty easy to use unless it's bent and not holding.

I plan on adding mucho downhaul and am afraid that the little "V" hook won't be strong enough. I guess that i could have a heavier one made though.

As far as the rotator, Murrays and Catsailor only sell the Hobie style rotator. I would rather use the front mount system that NACRA uses. It seems simpler to install and use. I dont have a grommet in my tramp like the miracles, and I assume thats what I would need to make it work.
Then again I could be wrong.

You can use the wishbone style off the front of the mast like the nacra one. If you preferr, call Steve Murray and I would bet they have one (Nacra style) floating around there somewhere.
If you're still running the Dacron main, there's only so much all that downhaul will do to flatten it. Start with mast pre bend first. If you go with an aftermarket main (Whirlwind or E/P) start with about 1.5 inches of prebend. If you get the prebend set for the sail, you shouldn't need more than 8:1 for that mast/sail combo. It's a pretty "flexy" mast to start with.