Aftermarket kits are available for the non-nav models. I have one on my '06 but it came on the car from previous owner. it was a non-nav model. Frankly I prefer the stock nav screen look to that of an aftermarket head unit, they all look and feel juvenile.
My 03 is a nav model. GROM sells a carplay/android auto kit that works with the factory nav models but I have not tried it yet but will soon. Some of those kits come with a new screen, but I don't think the GX one does. For what it is worth, I have one of these kits on my 2017 IS350 and it works pretty well. 100x improvement over the infotainment garbage from the factory.
Select Lexus GX470 2003-2004 VLine VL2 Infotainment System Upgrade Video Interface CarPlay Android Auto
Select Lexus GX470 2003-2004 VLine VL2 Infotainment System Upgrade Video Interface CarPlay Android Autogromaudio.com
Grom on the pre 07 screens is pretty ugly due to the low resolution.
I’m one of the suckers that bought a grom Vline 1 that was a buggy piece of crap for the first year, okay for a few, and then became a slow piece of crap until they totally broke CarPlay a few months ago and now it’s completely useless. I can’t recommend.