Navigate to the Heaven & Hell savegame folder on your hard drive. Click on this folder and open the MissionData.save file in Notepad.
If you are playing the Good side, change the settings under BaseState = hbsGood;
If you are playing the Evil side, change the settings under BaseState = hbsBad;
Find the entry that looks like this: EnergyCount = 2162; and change it to as much mana as you need. Save your changes, start the game and load your position.