I don't use LEDs for flashers, as their increased illumination rate doesn't buy you much, but a resistor should work. I'm not sure what the resistance of a typical blinker bulb is, but you can measure it easily enough with a multimeter.
For better or for worse, I also left the bike right-hand blinker bulbs intact. I figure it's more visibility if nothing else. We'll see if I pass NJ inspection if I ever go to NJ inspection