I bought some older suntour bmx pedals, you know the rectangular cage ones, off ebay a while back.
It wasnt till i tried to fit them that i found they were both left hand pedals.
So i got my friendly welder chappy at work to build up the thread with weld, then i machined a r/h thread on the pedal spindle.
Bobs your uncle, one pair of pedals.
Wouldnt be much different to go from 1/2 to 9/16.