Some dudes did pretty much the entire Korg line for free a while back and it’s not that hard to find online. I forgot the name of the developers but a little google search should point you in the right direction.
Another easy and great one is the OB-Xd which is an OB-Xa clone, also free, also pretty great.
I also recommend Helm, which is free. It can look a little overwhelming at first but if you get the hang of subtractive synthesis you will love it.