The A321 MFD draws that data from the default GPS.
I don't have FSX on my laptop, but seem to remember the GPS is a .cab file also, so you might find it in the main FSX Gauges folder.
Another option would be to see if you can change the MFD background colors which might make the text easier to read.
If you are not familiar with image editing I think you could edit the .bmp files with MSPaint
FYI, if you have both A321.cab and the unpacked A321 folder in the A321 panel folder FSX will use the unpacked folder before it uses the .cab. (I know FS9 works that way and am pretty sure FSX does too).
That being said, you can hack away at the unpacked folder without fear. If things go sideways, just delete the folder and as long as you still have the .cab you are good to go.
WWOD---What Would Opa Do?
Farewell, my freind (sp)