While exploring locations you might come across a broken vehicle that contains a rusty engine. Such engine can be also found at the "Junkyard" event. To get there, you need to have a level 40 bike at your disposal. There is not much you can do with it on the spot, but if placed on the recycler it can be disassembled into several useful items.
Please note that can only be brought home on a vehicle; it cannot be carried home by hand.