For as much Mana as you need simply save a game and goto the Programs Save Game Folder. Eg - C:Heaven and Hell - live and let DieSavegames. Your saved game will have its own folder. Click on this folder and open the MissionData.save file in Notepad.

If you are playing the Good side, then change the settings under BaseState = hbsGood;
If you are playing the Evil side, then 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.