WHAT'S THIS MUTAGEN STUFF DO AGAIN, BOSS?: At the beginning of the game, you may place adjacent a Warthog bystander. When the Warthog is KO'd by an oppoent's attack, for the rest of the game Bebop can use all of it's powers and modifies his combat values by +1.

IF I have more than one bebop do they all get the bonus.
When the warthog dies?

No.

The effect is referring to the Warthog placed by that specific Bebop character. This applies to both markers and generated bystanders. Core Rulebook, page 31:

Thanks much appreciated.