The save game file is in text format. Open the save game of your choice using Wordpad. Press Edit and select Find: The file is too big to manually get to where you want. Type "MOTHER" in the find box and click ok. Scroll down 10 or so lines until you find the lines that tell you:
FOOD
WOOD
GOLD
Edit the values just how you like them. Save the file and reload the game.
All Cheats
Without pressing enter, you can type in the following codes for their instant effect:
Code - Effect
DOYOUSEE-Reveals Map
GIMMEGIMME-10,000 of each resource
MAKEMENAILS-makes your troops tougher