If you wanna still get the PC version "legitimately" I guess
a steam key would work as there are like 5 quadrillion websites where you can get those from though security for some of them may be a concern for most of you reading this so I wouldn't blame you for not doing this method

This may be a bit hard or easy depending if you have an iPhone or Android phone as extracting it from an android phone is much easier
If you want more insight I HIGHLY recommend MegAmi's guide on extracting the rsdk file from the android version here
https://gamebanana.com/tuts/14942

as for making it mod compatible use the Mobile to Steam Conversion mod
https://gamebanana.com/mods/379103

If you somehow still had those console ports of 2011 Sonic CD and have jailbroken your console which most of you reading this probably haven't and have some sort of tool that can rip the game off of the hard drive of your console then you can hopefully get an Data.rsdk file that works with the decomp as I think the PC and console rsdk files may be no different

If you need to know how to install RPCS3 just google it
as for extraction just go to your dev_hdd0 folder then the game folder then the NPUB30624 folder then the USERDIR folder where you will find the rsdk file alongside the other files needed to play except the videos which are in PS3 .pam video format
You can also do it on the trial version and still get a full rsdk file out of it

as for the Xbox 360 version It'll be slightly harder. First get WxPirs, then open your Sonic CD X360 rom file and extract the contents to a folder of your choice and thats it!.

For those of you can't get through all of this RSDK extraction junk there is always the option of playing a version of Sonic CD that doesn't run on RSDK like the original Sega CD/Mega CD version via real hardware or emulator which opens up the door for Sonic CD++ which adds in some of those 2011 features like a better camera and spindash. You can also get the 1996 PC release of the game that can run nicely on Windows 10 via Sega PC Reloaded or playing the game on Sonic Gems Collection which is based on that 1996 PC port.

and to future proof this list an emulated version of original Sega CD version is coming on to the Mega Drive Mini 2 so that's cool