Uranos doesn’t really have a ton of feats. He’s Thanos’ great uncle according to Eternals #1. Thanos called him his grandfather but that was an error on the writers part. Kronos is and has always been his grandfather Uranos is Kronos’ brother.
Anyway in a straight brawl Mongul senior probably wins, but if we assume that Uranos has the standard abilities of the Eternals although greater in power, he should win.