Jasmine,
I was in Rome last March and stayed near Santa Maria Maggiore on the Esquiline Hill. There is an obelisk at the front and back of the basilica, which is huge. However, I was under the impression that it had been built over a temple to the Magna Mater, not Isis. I do not know if I read that at the actual church or in a guide book... I do have a great guide to archaeology in Rome, so maybe it was that book. Do you have any sources about this? I might've gotten the wrong info, so want to know the truth! However, Isis had a temple on the Field of Mars, and the marble from that are now distributed all over the city, the steps are on the Capitoline leading up to the Church of the Altar of Heaven... and what remains of her statue is outside the United Nations building!
I also went crazy trying to find the Temple of Isis remains in Ostia Antica!
Thanks,
Lupercus