Same here on spare parts - and faded control panel plus slide tables. Those parts unique to the Flat Top are going to be a really tough challenge, but as Larry noted earlier, the matching Summit parts are often available cheap - or even free - as those grills do come up for sale now and then. As Paul said, burners are currently still available, and I know from experience that Dave Santana/rcplanebuyer makes the flavorizer bars.
On the grates, I suppose cast iron or really heavy stainless would help with heat retention. On the other hand, as you can see in that Australian advertisement, at least down under they used stamped porcelain grates. With 50,000 btus in the same grate space as an older classic Genesis, there is no lack of power in these grills, but I don't have experience actually cooking on mine yet. I need an LP manifold and a Crocodile Dundee outfit first!