Do you need cross platform support on the server or client? Or are you looking for a standalone solution, as in one?
If you already have many crystal report templates, then i-net Clear Reports (used for i-net Crystal-Clear) may be the solution for you. You can use it to read and run your crystal report templates. I know that at present it is only Java. But on the server this should not be a problem.
Our guys are working on the .NET port. It should be available in a few months. Then he should also work on MONO. The big problem with MONO is that it is not completely. This will create restrictions for many applications. What do you want to do on MONO?