is it ok to replace only the valve seal?

my engine is still running great(strong and quite) and still looks great because of proper maintenance. the only problem is that since this February it started emitting white smoke once in a while, while driving. but now its continuous.

i'm planning to have a top overhaul, but just replacing the valve seal. i don't want it to be brought to a machine shop. because it still runs great.

any suggestions?

thanks