Search for Windsor to Romeo swap. Tons of people have done it (including myself).
Try searching car-part.com for available motors. Use a Romeo from a 2001+ Mustang. Even better, use a motor from a 2003+ Ford Panther (Town Car, Grand Marquis, Crown Vic). Using the 2003+ will get the PI heads with more threads in the spark plug holes.
For an absolute plug and play swap, it will be necessary to use a motor from a 1999-2000 Mustang GT. Expect to pay more for these Windsor motors as they are not as common as Romeo.
2003 GT Convertible (sold & missed)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.