You might have a lot of problems with fitting a naftermarket filer on the carb. As wayne rightly says, you use the centre bolt for fixing it, you can only use a cone filter with a jubilee clip on a weber carb, which has a much igger lip (and is crap anyway)
However, the ECC system has quite a few sensors in the airpan, and about 3 vacuum spriung valves too, so to do it all properly, you'd need to relocate those to K&N by drilling the base plate and mounting them al and wiring them all up.
If you just want the daft sound, just drill your air pan, but only the outer rim of it, i.e. the area BEFORE the filter.