Pausanias (6.12.6) comments that Hippomachos defeated three opponents without receiving a blow or any physical injury.
Moretti (1957), no. 506 claims that Hippomachos must have been the brother of Theotimos (ID 1098 in this database). Moschion, the father of the brothers, fought with Alexander the Great against Darius and the Persians (ca. 336-323 BC). The sculptor Daitondas from Sikyon made the statue of Theotimos and was active at the end of the 4th and start of the 3rd century BC (Moretti 1957, no. 489). Hippomachos' victory can thus be dated around the same time as the one of Theotimos, ca. 325-275 BC. Moretti suggests the 120th Olympiad (= 300 BC).
Persons called Hippomachos also appear in Aelian V.H. 2.6, Plut. Dio. 1, Ath. 13.548c, but it is not at all sure they are referring to the same person. [DvdL]