is it possible to modify a flasher to blink normally with an led turn signal?

wire in a 21 watt bulb in line under the bonnet to give you the required resistance.

not the best way but is the cheapest and will prevent the led flashing too fast.

try a timer circuit using NE555 ic that will drive a 12 volts relay. Oscillation can be adjusted

you can try a heavy duty electronic flasher, i use them in our government vehicles with the LED lights no problem.

Sounds like you need the low voltage converters installed inline.