a simple reduction ring with 43 mm inner diameter, to fit into the existing 65 mm or 80 mm spindle mount, like imagined here, would not help. Even a mount like the existing 65 or 80 mm Onefinity mounts, but now with 43 mm, would not help.
If you look at how the 43 mm mount milling motors are made, you see that such a solution would in most cases not allow you to reach your workpiece. This is because the Onefinity 65 mm and 80 mm mounts do not protrude downwards beyond the lower end of Z assembly. You can see this when you follow the thread to its end, where you finally have to land with something like this.