Tongue of the Fat Man

For invulnerability, use the DOS utility DEBUG. 
First backup your game to be edited, then type the
following from the DOS prompt:
ren disk3.ovl cheater.cht
debug cheater.cht
e 1ca7 7b
e 96be 83 3f 00 7e 07
Then type: w
        q
Then rename CHEATER.CHT back to its previous name.