Who were some sports stars in ancient Rome 44 bc?

It's hard to say. The few names that have survived are mostly on bits of grafitti and names on the occasional tombstone, which doesn't mean that they were or had been real 'stars'. The concept of a 'sports star' as we know it today did not exist then anyway. Many of the participants in 'professional' sports were slaves, most of them died before they even could get famous and in the total absence of any 'media', gladiators and such were at best known locally. People back then liked the fight, not so much the individual fighter. An obvious underdog might get extra support during a fight and get cheered at if he won against all odds, but that didn't make him a star in today's sense.