A small script that does not require CLEO. The essence is the same as in Timetwister for GTA 3. It opens access to any story mission, as well as opens all save points and the second island from the beginning of the game, and gives you a T-shirt for 100% completion.
P.S. The file is taken from the English-language forum, while downloading it still won't work because there is a broken link. I downloaded the file using web.archive. Mod author: Demarest (his nickname on gtaforums).
Installation: Make backups of the main.scm and american.gxt files and move them to a convenient location. Transfer the american.gxt file from the archive to the text folder Transfer the main.scm file from the archive to the data folder. Play. Screenshots with the location of the missions are available in the archive.
P.P.S. Rampage will have to collect luggage / jumps / missions.