I haven't found any hardened grease that I couldn't clean off with a little WD40, a toothbrush, a soft plastic scraper, and elbow grease.
ZEP citrus degreaser is fine on aluminum, as long as you limit exposure time. You can't leave Al parts soaking in it though, they'll turn black. It's also the best hand cleaner I've ever found.
I'm about to start refurbishing a 1958 Raleigh Record Ace Moderne, black with white head tube and transfers. The white is quite dingy, any suggestions for cleaning it?