It's not possible to re-generate the .sma file from a binary. There is a decompilation process that can be performed but it doesn't give you the original source code. You would still need to re-write the entire plugin using the decompiler output as a reference. It is often easier to write the plugin from scratch.
See
here.
P.S. Technically, it's not allowed here to attach any .amxx files. There are tools out there to allow you to decompile it yourself (one option is in the thread referenced above).
Linear Mode
