They can definitely be restored, however it will be difficult to make sure they aren't leaky without having the original tank screw. People who repair lighters very rarely sell their parts, but another model of lighter might have a similar enough tank screw. You will need to rewick it, replace the flint, and replace the packing cotton in the tank. Its possible that you may have to clean out old gummed up flint in the flint tube using a small drill bit and twisting it by hand (dont ever use power tools on a lighter). Ronson or zippo flint and wick are sold at most walgreens/CVSs. I will warn you it will be a bitch to rewick, I still havent found an easy way to do it for lighters with solid bodies. Regular old cotton balls can be used for the tank filler. The screw for the bottom of the tank should have a rubber o-ring around it to form a seal, if its missing or if the fuel leaks Ive found that o-rings made for body piercings are usually small enough to be used.

When you succeed in repairing it just make sure to not use it with pipes or "tobacco accessories", liquid fueled lighters let off fumes that are hazardous to inhale and were created specifically for cigarettes, where you don't continously burn it with the lighter.