Can I change Calendar Event details with AppleScript?
Shane Stanley has a library that allows you to modify the time zone:
https://www.macosxautomation.com/applescript/apps/Script_Libs.html



Update to my original reply:

In fact, your question prompted me to use the library and I found a bug in the time zone modification code, which is fixed now.

To create (or modify) an event you don't need Calendar. Just use this library.

The code here is almost 100% from the examples, I just added the line about the time zone modification:

use script "CalendarLib EC" -- put this at the top of your scripts
use scripting additions

    set d1 to current date
    set d2 to d1 + 1 * hours
    set theStore to fetch store
    set theCal to fetch calendar "test" cal type cal cloud event store theStore -- change to suit
    set theEvent to create event event store theStore destination calendar theCal event summary "A test event" starting date d1 ending date d2 event location "Around here" event description "some notes" without runs all day
    set theEvent to modify zone event theEvent time zone "Asia/Tokyo"
    store event event theEvent event store theStore
    return event identifier for event theEvent


You can find the available time zones this way:

use AppleScript version "2.4" -- Yosemite (10.10) or later
use framework "Foundation"
use scripting additions

current application's NSTimeZone's knownTimeZoneNames() as list


The discussion itself is here:
https://lists.apple.com/archives/applescript-users/2017/Dec/msg00006.html