To check for cylinder block corrossion,
Remove the cylinder head and see for yourself, if the metal is a solid casting around the cylinder bores, and the surface is smooth, you are good
If its eaten away and has compromised so that the sealing surface around the cylinders is very narrow (less than 4mm) the gasket wont hold, you need a new block. (And consider this a lesson to never rely on plain tap water as coolant)
The head will also will have been damaged, if the head has been “faced” again and again, throw it away as its trash now.