What is the obligatory amount to be given out from money gained from selling multiple motorcycles, with doubts about their stolen origin, and does the ruling on Riba (usury/interest) apply to it?

Mere doubt that an item is stolen does not prevent its sale or purchase. However, an item known to be stolen is forbidden to sell or buy, and it must be returned to its owner. Therefore, profit from selling bicycles is permissible, except for a bicycle that you know to be stolen, in which case it must be returned to its owner or its value be given in charity on his behalf.