You can, but it is not a drop-in. A 4090 Founders Edition will fit, but you have to remove the powersupply shroud and plastic fan bracket. Then get a 6-8 adapter dongle so you can convert the 2 power wires into 3 connectors to plug into the 4090 harness. The 750watt supply will work if you power limit the 4090 to 80%, will still get 95% performance out of it.   
I am looking to upgrade to a RTX 4090 card but it needs more than the 750 watt PSU in it. Can it be upgraded to a 1000 watt PSU? As long as I am asking, will the RTX 4090 fit in it?
Aurora 13, PSU 1000 watt, RTX 4090