Is it obligatory to repeat the prayer if one feels lustful during the prayer, without finding any trace of madhy (pre-seminal fluid) after searching for it?

You are not required to repeat the prayer, because the original principle is the validity of purification and its non-invalidation. What you are experiencing is severe whisperings (waswasah) from Satan to make worship burdensome, and you must stop them so that they do not lead to abandoning prayer.