Little update 
After seeing the kernel problem for the piscreen, I downgrade the kernel : sudo rpi-update e1050e94821a70b2e4c72b318d6c6c968552e9a2
And after my Piscreen is working as expexted 
On my Zynthian V2, I have screen, coder, soundcard working.
I test sound with test audio in admin, test midi seems to not work, same for engine but I don’t install all.
Now with the good kernel, I will try another installation.
To be continued…
Edit :
I have a working zynthian 64 bits, just fluidsynth and linuxsampler which don’t work, I don’t look for the moment why.
1 - download and flash https://downloads.raspberrypi.org/raspios_arm64/images/raspios_arm64-2020-05-28/2020-05-27-raspios-buster-arm64.zip
2 - Put in your raspberry, boot and configure your wifi and activate ssh ( https://linuxize.com/post/how-to-enable-ssh-on-raspberry-pi/ ) I skip update since zynthian script do it for me
3 - ssh into your raspberry
4 - sudo mkdir /zynthian
5 - cd /zynthian
6 - sudo git clone https://github.com/lguyome45/zynthian-sys.git
7 - cd zynthian-sys/script
8 - sudo ./setup_system_rbpi_raspbian_lite_buster.sh
9 - take many coffee, it take long time to do the compilation but keeo an eye on the screen, at beginning there is some question for example for the kernel downgrade
After the compilation is finished, just go to the webconf to configure the kit, maybe update the lv2 plugins and normally that’s all.
@jofemodo in my branch I make modification in update_zynthian_sys.sh and eynthian_envars.sh to change fb1 to fb0, it’s easier for me to have hdmi output.
I will test tomorrow with less modification.