While there is the popular OpenRCT2 for an open-source re-implementation of RollerCoaster Tycoon 2, it does rely on the original data files, and so the FreeRCT project aims to be a completely free and open source inspired game.
Very early days for the project yet but it has to start somewhere, and the idea is a promising one. It's not just a basic clone or a game engine reimplmentation of the classics, instead it's inspired by it to capture the feel. Perhaps with enough time, and enough people to help out, it might reach the heights of OpenTTD which is based upon Transport Tycoon Deluxe but doesn't require the original data files.
Will be keeping an eye on this one.
See more on the official site and download from Flathub.