In the folder CHARS under the folder CENTIPEDE was installed into, is the text file CENTI.CHR (example: C:\CENTIPED\CHARS\CENTI.CHR).

Near the end is the section FIRSTINVENTORY=
Changing all these 0 or 1 values to 999 will give 999 shots of all "special" weapons (SPACEBAR or mouse button2).

Down a little farther is HITPOINTS=
Changing this 1 value to 0 will make your character invulnerable.