Whether you are rebuilding the complete engine or just overhauling the Valve gear drive train - the ExhaustValve/S and associated parts should be renewed preferably replacing all the interfacing parts at the same time - mixing worn parts with new can lead to premature wear of the new item!
Have you ordered ALL the Valve springs, gaskets and seals you may require!
Don't forget the gasket sealant, try 08C70K0234M - for a more professional result!