I'm not sure if there's one. I just had a look at that github repo and forked it to my repo. Tried building it and it failed due to a missing "lib" folder. I have sent a pull request containing that fix. However, the build still fails at a later point due to various missing library jars (like Hibernate, JPA, Spring). I don't really know if Rafael has fixes locally which he hasn't probably pushed to the repo.
It looks like JForum3 combines some old lib(like Lucene, commons-lang) and some new (like vraptor 3.4), some deprecated features were used, so the exact version of jar files have to be there to make it compile.
I also see things like groovy were there, which sounds not mentioned in the design discussion thread. Do you know whether I can find an updated document for the current design?
JForum3 used a lot of new interesting stuff and code is nice and clean. If there is a full compilable version available, it would save a lot of time(like finding and downloading the exact jars) for anybody who want to contribute to the project.