How's your welding? (Mig, not ark). If the cracks are excessible without taking the manifold off, and your going to have to replace it anyway, why not try reparing it. Cast is finicky stuff to weld, but it can be done. Run the engine first untill manifold is as hot as it'll get. Weld the cracks in short bursts, not heating the meatal any more than nessesary. If you find your taking a long time to finish, run the engine up between welds to keep the heat disitribution on the manifold uniform. Once you've finished run the engine again to even out the heat over the whole manifold. Let it cool naturally, the slower the better. It'll either hold, or crack some more.
Like I said if it's knackered now, you've nothing to lose trying.
I have sucssesfully welded my trialers exhast pipe to the manifold, when the retaining studs split right out the sides of the manifold casting.