I completed this job on an older bike not long ago. Usually, the original flasher uses a bimetalic strip that heats up with the wattage used by the original bulbs. LEDs use a fraction of the wattage that bulbs do, so you’ll have to procure an electronic flasher, that is merely an electronic timer for your lights. Poke around on Google or go to SuperBrightLEDs.com, a company that specializes in LED conversion parts.
If you can’t find the electronic flasher as mentioned, you can add, wire in some resistors on the circuit to the signal lights. I have used 8ohm 25 watt resistors one for both the (+) and (-) sides and this increases the load that the OEM flasher sees and should do the trick. You can usualy find them at an eletronics/electrical wholesaler type place. I Lethbridge Hagen electric ordered them up for me $12 each. Cheers