If you used an aluminium flywheel it would be so light I would think as to give you a lousy tickover and may even stall all the time? it would be ok for quick rev pick up and high revs maybe like racing, but I would think it would be terrible for normal use.
Also aluminium wouldnt give such a durable friction surface as steel would so will wear away quicker.
Also if you substitute the dual mass flywheel for a single one you would find the engine feels very rough as the cushioning and smoothing out effect the dual mass flywheel gives wouldnt be present, it might seem like youve got a tractor engine in it.
Thats my thoughts on it anyway, others may have a differing opinion.