I want to try converting a cobrahead LED (I am a traitor, I know) but I only want to do it so that it looks like metal halide. I am trying to find a high power cool white LED filament lamp that is equal to at least 70w metal halide. Anyone know where I can find one if they even exist?

You are looking at ~50W LED to replace 70W Quartz MH or ~90W LED to replace 70W CMH. There are no filament LEDs of such power. There are some other form factors but for 50W+ (and not a Chinese joke that can't dissipate its own heat) they are inevitably a huge corn of heatsink fins. That might not even fit into a small cobrahead and would definitely have screwed up optics

Aw darn. Guess I have to wait a few more years...

Wait few more years for what ? LEDs are going to remain electronic devices that need to stay cold, while dissipating 10's Watts of power at a small point is going to remain a source of heat. Those things are not subject to changing. Any foreseeable advancement in LED efficacy won't cut the power of LED needed in half if thats what you are waiting for, so it will still remain few 10's Watts device for the same light output, with the cooling requiremetnts respectively

If we are talking about Quartz MH then use CMH, it allready is almost double the efficacy of Quartz in the low wattages

For the bulb size of an elliptical 70W MH the filament LED may dissipate about 10 to 15W or so (my guess). That means no more than about 18W LED. And that could be about 2200lm, so barely an equivalent of a 20W CMH or so.

When you want to stay with something that is supposed to be fitted instead of an ME or so, don't expect any higher wattage at all, the problem is just the overall size being to small for any higher power.

For a 70W replacement, you would need about 50W of LED's. For that power you would have to really look for some decent made COB fixture, where the complete fixture body is the LED heatsink. The 50W really needs that large surface directly exposed to the ambient air, otherwise it will overheat and fail soon.

Maybe devise a special streetlight design to have LED filaments assembled in kind of a reflector pointed at the road.

The filament format makes sense only for the (incandescent,...) retrofits, where you have no other surface for cooling into the surrounding air than the bulb wall itself (and which has to be transparent or at least translucent).
Otherwise the classic chip on a heatsink is way more practical (all light already just in one halfspace means way more efficient optic, the direct connection to the heatsink always means way better cooling than even the best gas fill,...).
So the only place for the filaments streetlights I may imagine are the decorative post top types.

Actually no, the optics with arc tube (and therefore filament too) source are better than with flat device source, as can be seen from the performance of al modern HID vs LED luminaires. Reason is, the light goig from the arc tube you can shape with reflector and lens/prism (while the reflector is te best), while with the flat source all the light is going forward and leaving only the lens/prism option, which is a glare problem with small sources

Still the power achievable with filaments is just way too limited for the application, unless you want 400W size lamp to replace what was originally 70W MH