Some progress on my side...
With a lot of googling, i figured out that the Wandel & Goltermann ANT-20 "Advanced Network Testers" are using the same types of Motherboard. They existed in a lot of varieties from 486DX2 (the one i want, because it's the same as the one in the SNA) up to Pentium II boards. So i searched on ebay for such a unit. Unfortunately you cannot really see from the outside the exact type - the only indication is the location of the ps2 connectors. So i bought one for 50 EUR + the same amount of shipping. Luckily it had exactly that 486DX2 motherboard, just with an addtional adapter board for the TFT/DSTN display.
Putting it into the SNA, i was a bit disappointed because nothing was happening on screen when the unit was turned on. Measuring voltages showed that the 5V power supply rail measured only 1.8V. Turned out that the old motherboard had blown the fuse in the power supply - replacing that one made a picture appear on the external screen. The internal EL displayed just showed the usual Zebra stripes. I played with the Shadow/Cache settings, but that didn't had any effect. I then noticed a 'Simultaneous Display on both screens' option. I enabled that option, and the EL display showed a picture. Unfortunately only until the next boot, now i cannot get a pictures any more, except the Zebra stripes.
The display also has quite a lot missing lines - not sure what to do about them. I never worked on such displays, maybe it's a driver chip that can be replaced, i just don't know.
The Spectrum Analyzer section seems to be fully working, which is the important part - i put in a 1, 10 and 20GHz signal, and it all looked good.