See Alien Syndrome HEX-code, HEX-code #2 here!

Unlimited lives
ALIEN.EXE
0000E0AE: FF 90
0000E0AF: 0E 90
0000E0B0: 27 90
0000E0B1: 0C 90

Stop timer
ALIEN.EXE
0000B7BE: FF 90
0000B7BF: 0E 90
0000B7C0: 01 90
0000B7C1: 0C 90

For cheats in this game, use the DOS utility DEBUG. First backup your
game, then type the following from the DOS prompt:

ren alien.exe cheater.cht
debug cheater.cht
For infinite lives, type:
e e1ae 90 90 90 90
To Stop The Timer, type:
e b8be 90 90 90 90
Then type:
w
q
                                        
Then rename CHEATER.CHT back to its previous name.