Pick any rectangle speaker you want. Go to metal shop and have "transition plates" made that cover the existing hole and provide you with a new one that is the right size for the new speaker. Paint the plate the same color as the ceiling.
I saw this done at a project with round speakers. At first i noticed it and asked about it but a week later you don't even know its there.
Avid Stamp Collector - I really love 39 Cent Stamps