how to ENABLE/DISABLE parts of an IP-core depending on the instance parameters??

You can implement what you are after using the generate callback.

Qsys/SOPC builder does not handle boolean and real valued generics in VHDL, and it also did not used to handle VHDL libraries correctly (in the sense that the auto-created components could not be simulated in Modelsim due to missing library statements). To get around these issues I use the generate callback to create my own top-level instance. In your case, you can have two different HDL implementations, and pick up the one the user selects.

Cheers,

Dave