The fork lock on my K3 (the type that pushes up from under the lower triple tree) has two Phillips head screws holding the cover on the lock cylinder, maybe you could remove these screws and remove the lock cylinder???
I got my keys for my 750K3 from a local locksmith, maybe he's got your key blank in stock too? Give them a call, D&D Lock and Hardware Wilmington, MA 978-658-2597 they can cut keys by code and I suppose they can ship overnight too. (can't everyone?)