can a crack in manifold be repaired ?

Usually not due to the heat difference in the metal and the weld, they just break again.

Some have brazed them after a good cleaning with wire brush on U-TOOB but who knows if it lasts .

If this is a stock manifold, like cast iron, welds will break again. But if it's any form of steel, welds will likely hold. That's evident in the manufacture of exhaust headers. But this might not be a crack. It might be a split gasket as the break closes with heat. But if it is a manifold crack and you're OK with a cheap fix, get a high-temp exhaust sealant, like JB Weld Extreme Heat.

Most likely by the time you pay for the labor to fix the old manifold you could have gotten a new one. Dollar wise and penny foolish! Jim