You must use an air hold and put air in each cylinder one at a time to replace them. Or take the head off (recommended) and change the timing belt also if it hasn't been changed. If you take the valve cover and shield and pull off the camshaft caps you can remove the camshaft and one cylinder at a time put compressed air in them, use a valve spring compressor and remove the two keepers on each spring, then take off the valve spring and replace the seals. One at a time and put it back together the way you took it off. Alot of people leave the camshaft in and do the caps and each cylinder seperately but thats up to you. With that age and mileage I would pull off the head, replace the timing belt and the head gasket so I could do the whole cylinder head on the bench. Then you could do a cylinder head and valve overhaul. Reseat the valves and clean em up. Thats it. Ok.