MAZDA RX8 REPLACE/REMOVE

At 30,000 Miles I Had To Have My Engine Replace By Mazda (still Under The 100k Warrenty). I Was Told Basically That I Do Not Drive It Fast...

I Have A 2004 Mazda Rx8 (automatic) That I Need To Replace The Engine In. I Was Wondering What Other Engines I Could Possibly Put In It...