Uhm that's not so obvious, considering that Namco iirc used a chip where you could specify custom waveforms. The SID has just square pwm, triangle and sawtooth. As for the main opening theme, I think the bass timbre can be imitated syncing two waveforms, or at most with a sawtooth or narrow pulse wave. For the melody maybe a triangle wave, or a slightly low-pass filtered square wave...just guesses, there is much room for experimenting, it's fun stuff :)