There's a full range of Merlin engined Spitfires available over at Avsim and SimOuthouse. Just search on YAS.

That SeaFire is amazing! Theres a number of P-51 Mustangs around if you look!

And, if you ever decide to consider any payware warbird options I highly recommend MAAM Sim's B-25 "Briefing Time".  It cost's $25 USD but all proceeds go to maintianing and operating the North Atlantic Aviation Museum which is on the property of Reading Regional Airport Carl Spatz Field in Reading, PA USA. I was their for the '03 Avsim Conference.  The flightsim model is exactly identical to the real bomber within flightsim limits of course.  But, since they actually have the real thing to go on and real pilots to test I gaurentee that "Briefing Time" is bar none the most realistic bomber aircraft ever made for PC simulation.  Also of interest is Wings of Power.  It's a collection of Allied bombers and German wonder planes.  Very accurate and complete cockpits, full system simulation, and accurate but very difficult flight models.  Difficult because they are accurate, not dumbed down for gamers.  The B-29 especially takes practice and going by the book or it WILL stall on a full load take-off.  B-17 is the best flyer in that package and the German jets are loads of fun but you gotta watch the temperatures on them. Very accurate modeling of early jets.  The add-on costs about $30 USD but you get like 8 of the some of the best wartime power birds ever done for simulation. Great value for a history and warbird fan.