To get up to 255 bullet clip, bullet (in gun) and medpack, hex-edit your save game at the position (in byte): save_file_length - 59. There are three numbers that will match the amount of bullets in gun, clips, and medpack. Edit this number and save your file. The maximum is FF (255) since each were stored in 1 byte.
There's another one. In address :length - 357. You will see two byte (when health is full) 02 A0, edit the A0 to F0 and you will be invincible (doesn't work all the time, though)