You can try this: http://ask.sqlservercentral.com/questions/71956/getting-the-source-code-from-a-clr-assembly

The compiled code is stored in the system tables. Decompiling it can be tricky. .NET Reflector from Red Gate may help, or another decompiler, but it may or may not be 100%. You'd need to recompile and test to see if it works correctly.

Disclosure: I work for Red Gate