Doorbell Options - Existing 2 Wire Battery Powered Doorbell

Hi,Im looking at upgrading a very old Friendland doorbell system that still works.It’s based on an external doorbell with 2 wires, which is wired back to a door chime powered by 4 D-Cell Batteries and no transformer.Size wise, I’m looking at the smallest possible compatible ring unit that can be used with a plug in adapter and chime.Any suggestions would be greatly appreciated.I’ve attached some pics below to assist.https://imageshack.com/i/po0zKSvJjhttps://imageshack.com/i/poRaYecojThanks