Find the default.cfg file. Go to GameData/indy/default.cfg and edit it using notepad. (Be sure to back it up first)

Change the file to read as follows:

pack:0
title:0
freescenedata:1
vsyncinterval:2
milessound:1
nocontrol:1
cheats:1
resmgrload:res.dat
fullscreen:1

Save the file and run the game.

You will now have access to the builtin cheat mode with the following cheats:

Unlimited Health
Unlimited Oxigen
God Mode
Unlimited Ammo