It's doable, but it's a lot of by guess and by golly, as you unwind the 6 volt winding, count the turns and then double it, but use a smaller wire to rewind for 12 volts, you might even add 10 or 20% with the idea that you may have to remove some of it when you start it and see what developed. When I did this with a Honda 80, some 30 years ago, the light was AC and the current/voltage was self limiting, you may find the same on your 75 250. As for a sequential signal, do a search here, there was something about it or link for some time back, but it needs DC. Another idea would be to wind your coil so it's floating and has excess voltage, rectify it with one side to ground and capture the energy in a capacitor and feed that into a regulated simple switcher buck circuit. Even easier, if you float the 6 volt and rectify that, you could capture that with a cap and use a switch mode boost circuit light the LT1270a, but you have an output that can supply the current, but since the boost circuit is very efficient, it may work.
Here's a sequential flasher from days gone by