I guess there is plenty of m42 50ish lenses. If it doesn't have to be m42, there is much more.
However, the Super-Takumar 1.8/55 i can definitely recommend:
Very nice build, for me a haptic dream. Optically also good. Rather small.
Rather sharp wide open. Nice bokeh. But, when closed to a certain amount, it shows the saw teeth circles which you may like or not.
Depending on age and coating a bit susceptible for flares on back light ... (however i love that).
Affordable to cheap.
There => https://goo.gl/oWoYfO <= are some samples i took with mine.
Whether it is especially good for portraits, i don't know ... i use it more as kind of "fun lens", yet. (see pics)
I also have an XF56 ... but the takumar including the "fat" adapter ist smaller and lighter.
