How about paying a professional calligrapher to do this? Depending on the number of cards to write and the costs, you can have them elegantly printed up (less expensive if the number of cards is high), or have the calligrapher actually write them for you. And then you can simply sign the cards. While a hand-written note is more personal, the essence of good etiquette is that everyone is honored and comfortable. If the the result is hard for them to read, and embarrassing for you, then that's the wrong way to go.
Miley Cyrus to perform at a Bat Mitzvah?
A Bat Mitzva marks the transition from child to adult in Judaism. It's the time when girls become responsible for their own actions; doing Mitzvot and not doing sins. Until age 12 it's the parents obligation to educate them. A Bat Mitzva party is a recent invention for those people who don't realize the religious significance of the occasion. Once you remove religion from a Bat Mitzva celebration then it becomes a party and you can invite anybody you like - just keep in mind that a Bat Mitzva is a religious date marking a girls obligation to start keeping the 613 Mitzvot.