Hello all,
It really isn't a new topic at all since endeavours for creating such expansion exist since a looong time ago.
There is a well documented hardware montage in this magazine from the time. The most current (and also well documented) experiment on that topic I know about is the Speak-SID board from Michael Wessel.
Given that the MiSTer is an amazing playground for computer breeding, I have tinkered a bit with Verilog and Quartus in the last weeks to see what I could get running. Since most of the component were readily available, as well as an existing portage of the excellent PlayCity expansion, it was just a matter of plugging everything correctly together. The only thing I couldn't figure out though, was how to plug the SID module from C64 Core and I had to go for the (relatively simpler 
) module developed by Thomas Kindler. As a result the SID chips are 6581 model only.
On the software side to see if everything was working as expected, I have adapted the RSX program provided with the Amstrad CPC International magazine and the SID player found with the Speak-SID expansion, which is based on a source code originally written by Simon Owen for the SAM Coupé.
So here also I didn't had to do much myself 
Hope that you enjoy.
I attach the .rbf file and the test disk.
If you want to tinker with this Core on your own, I have put the source in Github
) indeed, one being to figure out how to make use of the actual SID module from the C64 Core...