Im planning on doing the conversion myself when I get my hands on a bolex.
I would do like this on Ultra 16: make a mold of the gate and make it in a tough plastic, file out the gate and keep the old gate under my pillow.
I rather want to convert it to S-16 but Im not that rich to make someone else convert it :(
But I've heard that the shutter also needs to be filed a bit and some gears must be changed, that means one fault and your camera is gone.
Anyway If someone should try converting to Ultra-16 you could try my mold idea, then you can always change back to the old gate. :)