I'm adding this comment since you told about your intention to make an FPGA based replacement for 6560 (that would be very welcome for everybody us, not only for the VIC2020 but also because it's the only IC on a real VIC that can't be replaced with modern part and they tend to be scarce and unreliable nowadays).
I'm sure you already know, but just in case: Randy Rossi in on the final lap about a Commodore 64 VIC-II replacement
https://www.youtube.com/watch?v=SCReI0pmkSY
I asked him if he planned to make a 6560/6561 replacement too, and he answered maybe yes, or maybe somebody will fork his project and do the needed modifications to make a VIC replacement.
If you didn't do it yet, maybe you would contact him and see if the whole thing could be done easier than starting from scratch.
I'm adding this comment since you told about your intention to make an FPGA based replacement for 6560 (that would be very welcome for everybody us, not only for the VIC2020 but also because it's the only IC on a real VIC that can't be replaced with modern part and they tend to be scarce and unreliable nowadays).
I'm sure you already know, but just in case: Randy Rossi in on the final lap about a Commodore 64 VIC-II replacement
https://www.youtube.com/watch?v=SCReI0pmkSY
I asked him if he planned to make a 6560/6561 replacement too, and he answered maybe yes, or maybe somebody will fork his project and do the needed modifications to make a VIC replacement.
If you didn't do it yet, maybe you would contact him and see if the whole thing could be done easier than starting from scratch.