Best bet would be another head, but I seem to recall that Triumph specified dimensions and techniques for fitting seat inserts to both inlet and exhaust valves. As they overlap, this would apparently overcome a crack between the seats.
BUT, unless the machining of the recesses for the inserts got to the bottom of the crack, it could continue to propogate with the end result of either causing the inserts to become loose or the head to become sort of porous behind the valves. (As there is no water passage, or oil way, or any cavity under compression pressure behind the valves though, that should not be disastrous).
The ideal repair might be to weld or stitch the crack and then fit inserts, but if the crack is shallow you can probably get away with just inserts. Although you need an insert for the inlet valve on the faulty cylinder as well as exhaust, I suggest this is an ideal time to convert the head to lead free by fitting seat inswerts to all exhaust valves and higehr spec exhaust valves.
A secondhand head might be cheaper than a repair though.

Section : Cylinder Head

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