Hello,

PoW wallet is not going to sync since nobody still keeps PoW wallet working.

You cannot use old PoW wallet with PoS client since they are incompatible. The error you saw indicated this. Therefore you need to import all keys from old wallet into PoS one.

Which methods of importing keys from PoW wallet to PoS wallet from the wiki have you tried already?
This is the recommended and easiest method: https://github.com/eagleflies/noblecoin/wiki/Importing-private-keys-from-old-wallet.dat-using---salvagewallet-option.