Wow, I'm pondering the exact same issue and was about to start a new thread for some help . I bought a pair of Graupner Speed 700BB's for my Dauntless that I am itching to try out -- but these new motors are still sitting on the shelf.

I too have the Dumas hardware kit - with the 3/16" shafts spinning the propellers. Like the pic below shows, right now they are attached to 1/8" motor shafts (common Mabuchi 55x type motors). So finding couplers was no problem.

But the 700's have 5mm shafts. I've looked everywhere I can think of on the web and can't find a simple brass coupler for a 5mm shaft that is compatible with my current setup.

I did find a "collet for electric motor" on eBay for a 5mm motor shaft, but the other end is for a 0.187 flex cable, commonly used in fast electric r/c boats.

Surely one of the members on this forum has solved this one already , right?