Two things I would try... Clear CMOS, so it knows it hasn't got settings and put some basic ones in there. Then secondly, some Asus boards for OEMs were set up with a soft power up mode that required tapping the space bar on keyboard. You can turn that off after you get into CMOS setup, but first you have to get that far.

I did clear CMOS before with the on board jumper, and removing the battery.
Ordered a ISA/PCI testcard for an error code, hope that will help finding the issue, never tryed such testcard..
There is zero signal from the GPU. I think the bios did not init.
Will also replace the two 9636 mosfets.

After the new battery is in, it might have random settings, so unless you clear it then, you're trying to start with random settings. Particularly in the case of Dallas/Odin modules where the CMOS RAM is inside that module and the battery in it is holding whatever has ended up in that while being shipped around, hit by cosmic rays and being poked by airport Xray machines.

I got that testboard today, plugged in a pci slot it shows me a short on the 3.3V line.

POST cards cant show shorts, only somewhat signal if a power rail has some voltage in the ballpark. What you are most likely seeing is lack of 3.3V on PCI slot, which is normal. Early PCI was all 5V, transition to 3.3V happened around 1999.

start with basics. Multimeter/scope and measure voltages. 5V in ISA/CPI/simm slots. CPU Vcore and Vio.
Next is, or rather are Reset signals. Starting with reset on ISA slot pin B2 (between ground and power)
Then look under good magnification and toothpick at all PIIX3 pins, and in general carefully look the board over for missing/cracked small SMD components and scratches both sides.
Then measure Resets directly on southbrisge PIIX3 https://mark-ogden.uk/files/intel/publication … rator-May96.pdf pins 127 cpureset, 128 pcireset, pin 126 PWROK, pin 28 RSTDRV
Then scope needed to check if clocks gen is running and producing 14.31818MHz for PIIX3 and whatever the CPU clock is.
Then scope/logic probe on BIOS address/data pins to see if any activity is taking place at all.