Jon,
Inlets braze up nice (with ~600C preheat, keep hot whilst welding, then cover with fire blanket to retard cooling) but I'd not try this in exhausts.
There is too much thermal shock in use from exhaust heat/head expansion & contraction.
As it's way thin there now you can forget arc welding it.
If I had to do it (provided there is access) I would olde skool it as I was taught in the 60s:
Vee out the crack (a little, but not if it's real thin),
Preheat head from a cold furnace up to 700C,
Drag it out onto the hearth, weld crack with oxy acetylene and cast iron rod + cast iron flux,
Turn furnace off, push head back inside it to cool down overnight.
Deck the head to ensure it's flat.
Oxy welding cast iron heads with gas is a bit of a lost art now. It's hardly worth all this trouble unless it's an extremely rare head..
