To start, do you want a new Shemp, or a new Curly? Obviously, White wanted a new Curly, without spending any money. Besser was already under contract, it's said ( I don't quite believe that ), so no new money was involved with him. Buddy Hackett said he was approached, and though his story has been widely disputed, he might have been good. He was a good "nut" comic, and a self-starter, and the right shape, but really too young for the other two. Mousie Garner is a definite maybe, but so little of him is available to see that it's a tough call. Lou Costello ? One is tempted to say impossible, but he had broken up with Abbott, he was broke, and was just gigging around as a single. Also, since he had a history of heart trouble and had only two years to live, he may have been too sick to take a gig like that. How about Joe E Ross, officer Tooty from Car 54?
O K, a new Shemp: obviously, the back of Joe Palma's head wasn't going to work, how about the front? Nah, he was an actor and a stunt man, not a real comic. Shemp's physique was normal enough, so any mesomorph 5'5" or under could get greased down and step into the role, the only question being how much comedic spark the guy would have. How about Huntz Hall? Too tall, maybe, I don't really know, and he may still have been under contract as a Bowery Boy, although the timing on that might be split-second, and his contract might have been buy-outable. He had certainly ridden that horse about as far as it was going to go. Huntz apparently loved acting enough to continue, because he didn't need the gig: oddly enough, he was rich - he had invested his money very wisely in oil.
Here's my choice, I've written it before, and I still like it, although it's utterly impossible: Cy Schindell. Pros: he was a good clown, funny even when he was a villian, he had hair like Shemp, and he'd been with the stooges for so long and knew their timing so well that he was practically an honorary one anyway. Cons: he was a bit too tall, and he was dead. Too bad, that one.
We also have to bear in mind that the comeback lightning hadn't struck yet, and if ever an act looked like a dead-end career choice it was the stooges in 1957, who now had no more movie contract and whose product had stunk for many years by then. They had very little to offer any replacement, and looked at that way were lucky to get DeRita, who by his own admission was by profession a Burlesque comic and only dabbled, very much a part-timer, in Hollywood.