well your next move might depend on what special tools you have. removing the prechamber would provide better access than the exhaust manifold but in order to remove them, you need a special socket for the retainer ring and an adapter for your slide hammer. you also have to replace the prechamber seal ring. if you don't have the tools to pull the prechamber, I would suggest pulling the front or rear exhaust manifold (I don't think you said which cylinder you dropped it in) and rotating the engine over by hand until the affected cylinder is on the exhaust stroke. at that point the volume of the cylinder will be about as small as its going to get and the plastic bit should be towards the top so the vacuum trick may work. on the bright side at least you don't have a turbo that needs to be removed too!