But you say you don't have the original LLBLGen project file, correct?
In this case you'll start a new project from scratch (just make sure you take backup or that you use a source control).
Then using the manager templates, you may generate the code, and try to compile the solution. You might get compilation errors, because of manual settings were made in the original project, which you need to take care of.
But to save yourself a big time, I suggest asking them for the project file.