It looks like LuaDec itself was last updated for Lua 5.0.2. A new version targeting Lua 5.1.x bytecode has been released as LuaDec51. The wiki is always a good place to start looking for resources as mentioned by f3lix. The mailing list is also a good place to ask.
Do note that it is relatively easy to add obfuscation or even encryption to the bytecode loader. Neither would require edits to the Lua core. If the system you are analyzing has taken any of those measures, you might have more work to do.
