Have you tried "Advanced Scaling Settings": Settings -> System -> Display -> Advanced Scaling Settings -> -> Custom Scaling . I've never used it so I have no idea if it will work for you. Hope this helps.
I have two model M keyboards with PS2 cables, from the mid-80s, that I was considering selling one of on ebay. I have a 1390131 with the silver ibm badge and a 1391401 that works fine, but does have a gouge in the plastic front around where the indicator LEDs are. If you're interested, let me know how close you are to Seattle WA and we might be able to come up with a deal with the right offer.