Put Marquez on any bike and he will win on it. He is one of those kids who never gives up no matter how hurt he gets. He only knows to ride fast, all the time.The power delivery on the 500cc is in a very narrow power band and comes on brutally with much more power than a 125 has, it has a totally different engine characteristic to a MotoGP's more linear power delivery. Not saying Marc would be hopeless on one, but given his wild riding style it wouldn't be anywhere near as forgiving as his MotoGP is with its current electronics.
To compare MM and JL on a 500, I would say MM's riding style would be much more prone to highsides on a 500 than JL's .... I could be wrong.
Lorenzo would been very good on a 500. He was awesome on a 250 too. To be honest those who think only Rossi would win now on a 500 are a bit daft, I've heard it many times that he would dominate if they were on the 500. What a load of ..... Put the top lads on any bike and they will win.
Example, look at Marquez when he does flat track, totally different beast to MotoGP, yet he still wipes up most of the time