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.