Moral Delemma????
It is only a moral delemma or ethical delemma if you have no moral compass. Have you asked around to see if anyone knows the owner? If you find the owner offer to fix it up for them or offer to buy it. Come on - think of the other person instead of your own selfish desire to have what does not belong to you. Put yourself in the owners position (which you obviously don't know now) and for some reason you had left your bike out like this one and became somehow unable to give the bike the attention it needed and some jerk came along and thought his desire for your bike was more important than your situation as the bike owner. Do what is right! If everyone did what is right in his own eyes we'd have anarchy. And that's your ethical delemma?