Make yourself an inexpensive female casting off the original spline. It only needs to encompass one tooth and two tooth spaces on the male shaft spline. Auto body filler seems to make a good enough stable casting media. When it hardens, remove it and form grind a flycutter to fit the space between teeth in the casting.
AFAIK, there is likely no really good drawing data for an involute since the shape is generated by the hob, which itself has a much simpler shape. But most of us don't have the luxery of a hobbing machine, so we make do with form ground cutters. I just grind them offhand on the bench grinder.
Then flycut the new spline out on the mill using the cutter.
The method that ClimatePro outlined, while I've heard of it, it does not really have any control over the pressure angle of the spline drawn. I'm not sure what the relationship is between this "string-generated involute" and the pressure angle. Does anyone?
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)