The RC3403 was the original quad op amp used in the OD 1. It's been out of production for a long time. You may find one on E-Bay, but a lot of the ones you see are fakes, and a real one goes for a high price. Any 14 pin quad op amp will work as long as the pin out is the same. The MC3403 is a good choice, and the TL074 isn't bad either. But if you want to get as close to original as you can, try the NJM3403AD, it is the only one that is ground sensing like to RC3403. As this is this one of my favorite circuits, I've built quite a few and have experimented with it quite a bit. The best sounding ones I've built used the NJM3403AD, carbon film resistors and the green Mylar capacitors. Also alot of prints and clones show the feedback potentiometer as a A1M, the original used a B1M. Either one sounds good but there is a difference in the feedback response and a small tone change between the two. Experiment with them and use the one you like best. If the PCB has the clipping option on it, LEDS sound real good as the second option or Ge diodes work well also. Look for a high fwd voltage Ge diode though, this has a very noticeable effect on overall output of the circuit. Youll notice a large drop off in volume with Ge diodes, bit the warmth of them shines through.