Well, you have to get it out, so when you do take it to a bolt/screw store and match it as close as you can. Or visit a wrecking yard. It's not a high-stressed item, so just about anything with the same thread pitch/size/length should be fine.
The book shows an early and late version, M8 by 29mm - 21 Nm torque; and M7 by 26mm, 16Nm torque. If your rotor holder that it screws into is aluminum, it's the 1st version; if steel, it's the 2nd version. But no thread pitch is given in the book - so gotta get it out and check.

DG