Thank you for the replies people.
I've thought about the landing craft and I think that if there were enough for the Sicilian landings then there should have been enough for Normandy landings. Also the Allies organised the Operation Torch fleets sailing out of the US and UK. They could have organised landings in Normandy originating out of the Med and lesser out of the UK and US. If they could build up half a million troops in Sicily then they could put the same number into Normandy and turn it into a fortified zone. The Allies wouldn't have 10,000 planes in support like in 44 but 4,000 would still be many more than the Luftwaffe.
I don't know if they would have liberated France and Belgium like in 1944 but the German Army in France in 44 was much more powerful than what they had there in 43. It had 2000 Panzers. In 43 it was probably a few hundred. So Hitler would have had to make massive transfers from Russia to contain the Allies in Normandy, much of the Kursk forces, and it would have sped up the Soviet 1943 advance considerably. Allied fighter planes flying out of Normandy could also have escorted B-17 bombers deeper into Germany much earlier.
Altogether I think a Normandy rather than Italian campaign would have been a far worse problem for Germany as the power needed to bottle up Normandy, the Panzer forces needed, would have left German forces in the east desperately weak.