Is there an off the shelf converter / adapter from C128 RGBI to VGA that will work with modern-ish low voltage VGA?
RGBI to RGB only needs a couple of diodes and resistors, see e.g. here, and that will work with VGA-RGB. I am not aware off-the-shelf ones for that are available, OTOH they are not exactly difficult to make yourself.
I have no idea what @mnem means by "RGBA", usually this is an abbreviation for red-green-blue-alpha, and refers to how image data is stored on memory. There is no alpha channel when outputting this.
Here's the RGBI to VGA RGB conversion circuit:
Credit: Alex Hajnal based on circuit by Frank Buß
The circuit above was based on the one shown below (from this page). Personally1 I'd go with the former circuit.
Credit: Alex Hajnal based on circuit by H2Obsession
1 Personally == @AlexHajnal's opinion