There is another thread that you should look at:
This covers an Adafruit 3.5" and changing the GPIO pins. There are a few other threads covering different displays but most require extra work on your part.
Not sure how much I can help, but the small advice I can give is you are better off with an LCD that is already supported by the current kernel and I ended up with the 2.8" model as per the default build. Do research and try to find out if the modules/drivers for the LCD are part of the current default Raspbian kernel. If that support is there, you will still have to edit Zynthian files to use it but
If your supplier’s support is a modified kernel or patches/drivers, beware: you will have to do extra work to get it running. This could be OK but then you are doing a lot of work/learning to recompile and install the kernel. And when the Raspbian main kernel is updated, the LCD will stop working. So then you have to see if your supplier has an updater kernel or revert to the older modified one.
I went down that road with a 3.5" KeDai; didn’t research it before buying. Turned out the display is really sluggish and they only supply a pre-compiled kernel ( and some fear there is spyware embedded in it). SO I ended up getting a 2.8" Adafruit clone (Ebay for $14) and it’s fully supported in Raspbian (and the Zynthian SD img too).
Waveshare LCDs are good judging from posts here, but you may have to adjust the config file to match the GPIO usage. If you have a lot of Linux experience or really want to dig into learning, you can get some of odd ball LCDs working but IF you are new to the Linux world I would stay with the sure fire/tested devices. Far less headaches 
Yogi