Hi lancashiregolf,
valve stem seals can be replaced with the head still on the block, easier with the head off and you will be replacing the head gasket,best to also replace the timing belt and tensioner while you are at it. Worn stem seals will make the engine smokey but not misfire, there may be another problem, ht leads and distributer cap and rotor may be worth a look.
Hope this helps,
Steve.