normal orings should be fine for those, just make sure there slightly fatter and slightly smaller in diameter to the new gasket.. but the smaller oring needs to be a loose fit on the stud or it will block oil to the head..
but if there not too hard you can re use them.. try soaking in boiling water to soften a little..
but having said all that, if i come across them and there hard i dont use them again and just use something like hylomar gasket sealing spray.. works wonders and shouldnt leak if all surfaces are clean and flat..
just be sure to check/re-tension your head stud nuts a couple of hrs after running the engine