Try this: fix the flats, pump up the tires, tighten the bolts, clean up the frame, spray WD on the chain--whomever it belongs to should appreciate your efforts: if the owner never steps forward to claim it, then you may be stuck with it. Then, give it to charity and take the charitable deduction on you taxes based on the market value of the contribution of the vintage bike, which you own at that time, due to the uncompensated work that you put into it's restoration.