I believe that ANSI B29.1M is equivalent to the ISO sprocket standard, meaning that the 6.35 mm sprockets are designed to be interoperable with 1/4 in (#25) sprockets. That means you can just search for ANSI sprockets, which are far more common.
Also, for the bore, if you can find something smaller than 8 mm, would you be able to enlarge the bore on your equipment? 5/16 in bore would be especially good, because 5/16 in = 0.3125 in, which is close to 8 mm = 0.3149 in, so you could ream it.
The keyway is a sticking point, though; without the right tools, it's not easy to make a good one. I've used an awesomely ugly setup involving an end mill ground into an approproiately wide chisel, and inserted into the quill of a milling machine (
), which was then moved up and down (manually) as the bed was slowly indexed. It's ugly, but lacking an arbour press, the machinist who owned the mill suggested this.... More recently, I've used an ancient EDM machine for the task, with mixed results (it's slow, and the setup has to be very accurate); in any case, EDM work is probably not an in-house operation. If you're desperate, you can file it, but be very diligent with the setup process. It would be best to transfer the outline of the keyway to the sprocket on both sides, so that the ones doing the filing and cutting can make sure that the keyway is not skewed (which is just about inevitable, otherwise). This might possibly be done by printing out a scaled drawing of the part, and overlaying it to make scribe marks, or by aligning everything in a fixture in a mill, and using the readout to find and mark corners, then scribing lines.
In any event, what you've found is hardly ideal, but I don't think you're going to easily find anything better, at any price.