It is true that the cylinder head is the biggest power restriction on the 200. However, a new head alone won't put out as much as it could unless paired with sufficient induction, cam, and exhaust. And all of that is as nothing if the bottom end won't hold up, though that is not a huge problem since they are very sturdy.
Do you mean the best stock head made in the US? That would be the D7xx series and up, manufactured from 1977 to 1983. Hardened valve seats, the largest valves, and the biggest intake volume. They also have large cylinder volumes and need to be milled to get a better compression ratio. Mustang Geezer has done most of the tricks that can be done, and has a calculated 130-160 hp. I believe Mraley dynoed at around 130, but had DUI ignition and a 2V carb, as does Mustang Geezer.
Of course, there is the fabled Oz head, that comes in several flavors. All are better than the US versions and can be adapted to a US block with varied ease. I believe the 250-2V put out 170 hp or more in stock trim, and the Crossflow put out higher numbers.
The Argentine heads are also better, but seem to be rarer here and less easy to adapt. By all accounts, they have the best power of the non-crossflow heads.
And, of course, the almost-here Classic Inline aluminum head. I should think that it's the ultimate, and should be available in the next couple months.
Disregarding, all of that, a turbocharger is by far the easiest way to gain horsepower. Linc's 200 ran a 13.58@103 in the 1/4, and that calculates out to roughly 237-256 hp. Totally untouched engine.
