Do you have a wheel arbor with the machine?
If so, find a couple of bearing balls that are close to the maximum and minimum diameters. Put them in the arbor and measure their distance from the top with a depth micrometer ( or caliper, at need). correct for half the diameter of the balls to get centre distance, then the taper per side is tan⁻¹(((dia1-dia2)/2)/difference-in depth-between-centrelines)
You can use the same calculation, with a bit more fiddling and swearing, by using rings with a known bore on the spindle.
For a perfect fit, once you’ve turned up a female taper, use spotting blue to see how good the fit is and minutely adjust the angle to suit.
Good luck 🙂
PS:- the grinder looks as if it’s got the best bits of the B&S No2 and the Jones&Shipman 540 combined.