My Matador, which is a10cc single, uses piano wire pushrods. This can just about be filed while rotating in the lathe. As the guys have said, the rounding only needed to be approximate to cope with the slight change of angle as the rocker moves. In the UK straight piano wire is most easily obtained from aero modelling suppliers.
Silver soldering a ball on the end will soften the wire and you really need to retain the stiffness. Whether Loctite can deal with percussive loading is up for debate.
HTH
Rod