I'm testing to see how old versions of Messages would run in new versions of macOS, and to get the apps to function, I would need the proper frameworks from the same version. I got my hands on the Messages beta installer for 10.7 Lion to see what frameworks it needs to install and use to function, and I need to know if replacing, modifying or deleting any of these frameworks will cause major system instability or break core system functions. I don't count iMessage or FaceTime as core system functions, so if replacing these will break FT, I won't mind. 32-bit compatibility is also not a problem, since I am running Mojave.
Here are the following frameworks/files I intend to replace or install:
/Applications/Messages.app
/System/Library/Spotlight/Chat.mdimporter
/System/Library/PrivateFrameworks/XMPPCore.framework
/System/Library/PrivateFrameworks/PhoneNumbers.framework
/System/Library/PrivateFrameworks/MMCSServices.framework
/System/Library/PrivateFrameworks/MessageProtection.framework
/System/Library/PrivateFrameworks/Marco.framework
/System/Library/PrivateFrameworks/IMCore.framework
/System/Library/PrivateFrameworks/IMAVCore.framework
/System/Library/PrivateFrameworks/FTServices.framework
/System/Library/PrivateFrameworks/FTServices.framework
/System/Library/InternetAccounts/iChat.iaplugin
/System/Library/Frameworks/InstantMessage.framework
/System/Library/Frameworks/IMServicePlugIn.framework
/System/Library/Frameworks/IMCore.framework (alias)
/System/Library/CoreServices/Menu Extras/iChat.menu
/Library/Scripts/iChat/Mix Message Case.applescript
/Library/Scripts/iChat/iTunes Remote Control.applescript
/Library/Scripts/iChat/Auto Decline.applescript
/Library/Scripts/iChat/Auto Accept.applescript
/Library/iChat/PlugIns/Yahoo.imserviceplugin
