Alternatives to a 4.50/4.00-12 inner tube tire?

i have a chinese knock off mini harley that i am trying to fix up and either sell or give to my nephew. The valve stem recently broke off. ive tried looking for this size inner tube all over town but haven't found one and haven't been able to find one online either. Is there a comprable inner tube that will work? Can i get a slightly larger inner tube and put it in there? I would assume the pressure would build up against the wall of the tire and fill the void properly even though it is slightly larger. Any ideas?