Do you have the classic Revell kit, or the Monogram rebox?
The Monogram kit includes a D-21 drone, and is the more readily available of the two. If no one responds, I have the kit and can scan the instructions at the office on Monday. It’s a pretty common kit that many others should have in their stash.
As an aside, Caracal has recently issued a decal sheet for the SR-71/YF-12 family, and is recommended.