Manifold heat control valve replacement?

You might be able to free up the valve so that it works. It's tempting to grab whatever penetrant you happen to have around, but do yourself a large favour and buy a can of this. I don't know what the heck is in it—it smells way different than any other penetrant, and it works differently too, including standing you your best chance of freeing up a stuck manifold heat control valve, which is what it was originally formulated to do.

Don't bypass or otherwise disable the valve; it serves a very useful purpose no matter where you live (hot, cold, whatever). This 1935 Jam Handy cartoon, "Down the Gasoline Trail", would be a real trip to watch even if it didn't contain a sequence specifically explaining why (at 5:37).

There used to be manifold heat control valve repair kits containing all the parts needed to rebuild the valve. They have been unavailable for years now, and NOS units are expensive enough that it's better to just buy a complete new manifold.

New manifolds are available, as 65 Dartman says. Quality varies quite a bit, so you have to shop carefully. Info on picking a good one and the necessary carb choke adaptation is here.

It is very much worth your while to get the good gaskets for the manifolds-to-head and intake-to-exhaust junctions, and while you're at it, pay attention to the mounting hardware