I'm running an '06 EC200. I use it for riding in sportsman / novice class and occasional greenlaning. Going to the single base gasket made a big change to how the bike runs, although I would echo Jakobi's advice about getting the tolerances and port timing right if you would like a 100% job. Just doing the base gasket was good enough for my purposes, although I might go to town on it one day. Fuel efficiency is about 80 miles to a tank and power delivery is spot-on for me, plenty there when you need it and doesn't punish small throttle control errors like the EC300 did.

The UK EC200 nearly always have 2k3 (believe other regions get the 2k2), so already have the bigger flywheel. I wouldn't personally bother with the powervalve cover, just freshen up the engine, build it up with one base gasket and go from there.

Proper jetting is the fisrt stop.

If you are going to mess around with gaskets, just get the bloody head done, it's cheap, any body with a lathe can do it, just make sure you adjust the compression ratio too. There are numerous posts telling you how to do this, or if you really need, ask me and I'll make a guide.