If Sigilyph with the "Toolbox" Ability has Plasma Badge and Reversal Trigger attached, do I get to search my deck for a card when it is KO'd by an attack?

Yes, you can. The tools are still present on the Pokemon when it's KO'd (assuming the attack does not discard the tools first), so you would get to use the effect of Reversal Trigger since Plasma Badge makes Sigilyph a Plasma Pokemon.

Relevant cards