You'll need to remove the radiator, engine fan, crank pulley, distributor and timing cover. It should be a rope seal, but you can carefully remove the carrier for that type, and fit a lipseal. If it has an SD1 timing cover it will have a lipseal already. So you'll need the seal, a timing case gasket and a sump gasket. Whether you use just the front of the sump gasket, or remove the sump to do the job properly is up to you. It's likely that you'll need to prime the oil pump, so you could add an oil pump gasket and some Vaseline to the list and prime the pump while the timing cover is off. If you mark the position of the distributor and the rotor arm and don't turn the engine over while it's apart it should all be in the same position as before it was dismantled.
Most oil leaks are caused by blockages in the engine breather system, so make sure that is all clear, otherwise the new seal will leak as well, or the leak will just move somewhere else.