Overhauling the cylinder-head assembly?
Renewing all the wear components at the same time will attain the best results. Valves, springs, cotters, rockers, valve/camshaft drive train and tensioner/guide will all help the engine give its best in a mechanically smooth quite manor.
Interfacing used parts with new will lead to premature wear of the new parts and compromise long term durability?
Other related cylinder-head and ancillary parts you may wish to renew: