During game-play, hold down TAB and hit the page-up key simultaneously. Then let go and press the letter P. At the bottom of the screen it will say "cheat enabled." This will make you indestructible. This won't help on missions where you have to keep other things, like the War Drum, from blowing up, but it will help in those nightmarish combat situations.

Or you may try this alternate cheat/hack:

HexEdit the named 'dl.cfg'
Goto offset 00000358
Change that byte to 32 hexdecimal
Save & exit.

You can now choose any mission you want.