Thanks to everyone for their input.Regarding the 2 stroke vs 4 stroke debate,I've decided to go with the OS 1.20 Ax.It cost the same as a ASP 120 4 stroke,but it puts out according to the published figures 3 Hp ,swings a 17'' prop at between 9000-10 000 rpm, thats in the the same range as similar 4 strokes but with twice the grunt!!!! Soundwise it certainly won't be an annoying little screamer.The only other concern I had was with the great little mufflers that 4 strokes have.They would certainly fit into the confines of a P51's cowl better than a 2 stroke's bulky item. What ultimately sold me to the OS120 is the fact that you can swivel its muffler 90 degreed to make a Pitts style approximation.Can't wait to pick one up. Regarding that troubsome MDS, I immersed the muffler in a cup of fuel,closed the barrel all the way and blew through a fuel tube into the venturi. You should have seen the bubbles!!!! As a stopgap until I get another muffler I Smeared silicon onto the barrel and that should help stop the leaks. Don't think I'm fretting over this engine.I've had it ten years and solely for my personal satisfaction I want to see if it is possible to tune it.It is on a test stand, as I would'nt trust it in an aircraft.