I spent 4 days in the forest, didn't find a Pikachu.
Spent an hour in the PowerPlant and I found 2.
I believe there is a better rate in the power plant, so try your luck there.
I've probably found like 15+ Pikachu by now just by hanging around the Power Plant.
I found 2 shiny pokemon, a pinsir, and like 4 pidgeotto in the forest for the 4 days I was there.