If it is a plain cast iron cylinder liner, you may be able to recover the cylinder by honing - you can find out just by inspecting it. The problem isn't the aluminium that has stuck to the bore, it is the scrapes and scratches, and those may be quite shallow. If they are shallow you can use a standard piston, which is likely to be more easily available than an oversize one. If it didn't get to the point of seizing, I'd guess you have a good chance of just needing a piston. Even the original rings may be recoverable. Are there other engines that use the same piston? If there are, you have a better chance of getting a second-hand one.
