If you really love your Flex, then look for a Mutar 1,5x. It gives you a focal length of 110 mm with the 75 mm Planar and slightly less than 120 mm with the 80 mm Planar. Please take care to get the right bayonet. Mutars are all equal, but the bayenet rings can be changed. For Bay II exits two versions, 42 and 45 mm lens distance.
I have both Mutars. They work fine with the Planar if you stop down to f/8. For headshots you don't need the last bit of sharpness in the corners, so that f/5,6 may be sufficient.