There was this sequel too
https://archive.org/details/More_BASIC_Computer_Games
I don't think I ever saw source code for Oregon Trail As for text adventures those require fairly exotic programming techniques to pull off. Infocom had the Z machine interpreter written in assembly for many machines but Scott Adams made an interpreter which could be implemented in either BASIC or assembly.
A simple adventure with a rudimentary VERB NOUN grammar can be done as a modest BASIC program, as for example in the Usborne Adventure Programs book https://usborne.com/gb/books/computer-and-coding-books https://drive.google.com/file/d/0Bxv0SsvibDMTYkFJbUswOHFQclE... .