What motor is it? If it has a 5mm shaft forget it. I looked everywhere for a 5mm to 1/4 and ended up making my own by putting a 3/16 collet in a lathe and running a 1/4 end mill through. Just have to make sure the collet sleeve is on so the inner jaws dont just expand over the endmill. Worked great for me.
I use these on mine, well made and you can get any size/configuration you want. If he doesn't have it, he will make it. I use the 'Big Dog' couplers on my 1/8 scales 8mm to .250.