HEART OF DARKNESS, SMILER WITH A KNIFE, and WAY TO SANTIAGO are all in the collections at the Lilly; HEART is actually also held in Indiana's general library on microfiche, so anyone can get a copy that way, as long as you don't mind feeding a microfiche machine for two or three hours. Huge fun, I can tell you. How good are they? SMILER, I've never looked at, so I can't say. WAY TO SANTIAGO has some fun moments and is very much in the anti-fascist mode Welles was working in at that time, but it also has some plot flaws that would have had to have been corrected before it got made. HEART is the best of the bunch, and it's really too bad it didn't get made. The others on the list, I'm not sure if script material survives.
SANTIAGO was in the news a few years ago, as some here may recall, when the revived RKO dug through a bunch of old scripts and found a copy. It was hailed as a "lost" Welles script and there was talk of making it. This was highly unlikely, given the re-writing that would have been needed, in addition to the subject matter (focusing on a Lord Haw-Haw type character, which many audience members might be unfamiliar with). The project unsurprisingly went away.