You could definitely use a pendulum to run an electric generator; however, all energy is conserved, so any energy that is gained by the generator is lost by the pendulum. So you could power a generator, but then the pendulum would gradually slow down and stop. If you then wanted to get the pendulum moving again (say with magnets like you proposed), then that would take energy.
Because all systems are not perfectly efficient (for example, there is the friction of the pendulum moving through the air, and friction between moving parts), if you used a pendulum to power a generator, then used the generator to power the magnets to keep the pendulum swinging, energy would gradually be lost, and the system would eventually come to a complete stop. You would need energy from the outside (such as a push every now and again) to keep the pendulum swinging.
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