I’m apparently not allowed to post links, so I’ll just say that it’s on Modrinth and that it’s the very first result in a Google search for “ars magica 2.5”. It seems like a direct upgrade for the most part and it has better NEI support (requires the GTNH branch of NEI, normal NEI makes it crash).
There’s also a fairly recent AM2 addon on Curseforge called AM2Playground and it’s pretty cool, however it is in beta and very unpolished so I do not recommend using it in any serious gameplay for now.