Can I apply for Citizenship while I'm waiting for the Replacement Green card or do I need to wait for the G.C ?

Good News. You do not have to wait.

You will find on page 5 of the N-400 (Naturalization) instructions for the form, that you may submit:
a photocopy of both sides of the green card I-551 (you should always have one on file in case of loss, etc.), OR evidence of your entry documents into the U.S. OR your receipt notice for the filing of I-90 Application to Replace Permanent Residence Card.

Of course, if you have ever committed any criminal offenses, drugs, guns, battery, fraud, immigration violations, or have long absences from the U.S.-- then these are separate issues that are critical where you wouldn't just go ahead and file-- as you could be rejected for a lack of Good Moral Character-- and it's 5 years until you can reapply-- or worse-- the wrong scenario and a person could end up meeting the Immigration Judge for Removal Proceedings. Just saying' As your question is straightforward... but that's the paperwork and filing-- you also need to be CLEAN and CLEAR to be eligible and recommended to proceed. You will find most screening questions on the N-400 form. These are there in part, so that should you deny one, such as have you ever been detained? Then, the police record in the adjudicator's hand will lead him/her to find lack of Good Moral Character for not being forthcoming about that overnight stay in the slammer.
Good Luck.