What is it obligatory to do to atone for the mistake of hitting a motorcycle and fleeing the scene of the accident out of fear of the consequences of not having insurance, while currently being unable to ascertain the fate of the riders?

The hadith states that if the accident was caused by the cyclist's sudden turn, and you were abiding by traffic laws, then there is no blame on you. However, if the accident could have been avoided, or if you exceeded the speed limit, then you bear responsibility. If the accident resulted in the death of two people, then blood money (diyah) is required for each deceased person, and expiation (kaffarah) for each (emancating a slave or fasting for two consecutive months). As for your departure from the injured and failure to provide assistance, this is a grave error, and some jurists consider it a cause for liability. You should strive to ascertain the condition of the injured and pay appropriate compensation. If that is not possible, then sincere repentance, increasing good deeds, and giving charity equivalent to the blood money are preferred.