Wow that would Murder in an FC....
eBay Motors: JDM Mazda RX-7 RX7 FD/FD3S 13BREW 13B-TT Parts Engine (item 110019303889 end time Aug-16-06 19:00:00 PDT)

 

the problem with those engines is the mounting. Instead of getting a 3rd gen motor, you just get a 13b-re (same thing pretty much but it's from a cosmo so the mounts are in a better location). 13b-re~$1600, ported rebuild $1300, mounts $300, ecu $900. Then you have one badass engine, some people argue that the stock twin turbo setup doesn't fit, but infact it fits just fine.