Psilocin with the alcohol group replaced with biosteres.
4-SH-DMT
Shares charge. Sulfur is more nucleophillic and bigger, would still have to fit in 5HT2A receptor pocket. Metabolism will be different due to oxidation potential of sulfur. Likely smells quite bad.
4-NH2-DMT
Swapping the alcohol for an amine might change its pKa too much and interfere with its charge at physiological pH, making it not cross membranes well.
Both of these were chosen because they would maintain the H-bond donor/acceptor character of the alcohol in psilocin.