I played this game with my friends in grade school on the PET.
Though unable to find the original, I found a stripped-down VIC-20 version -- which showed me the program's essential structure -- and the later 1986 version by Commodore, mainly designed for the Commodore 64. I used both to create this version, which retains the original algorithms, and tries to emulate the older PET version.
It uses very little X16-specific code -- exactly, SCREEN and COLOR. It relies on the 40 x 25 screen, and it uses just about all 25 of those rows.
The rest is all PETSCII and BASIC 2.