Hey Vito, welcome!
As for the resistors, you "might", but I'd try one of the newer flasher units first, like G's Baja mentioned above.
You should be able to get one that would work at any local auto parts store, but I'm not sure if it would work with just 2 LED bulbs or if that makes a difference.
I would also make sure that, when you have the lamps apart, you clean up and paint the reflectors with a glossy bright white paint. This gives the light more chances to fill the whole lens, rather than just be one small round point of light behind the lens.
And to that end, try the bulbs that have side-facing LED units too. Or some with very wide beam pattern LEDs.
Using one of the type that simply faces the LED's straight out will end up with just a circle of light.
Not as nice even as standard bulbs in that case.
Paul