Pilotman: This is a long one, but it's information you'll need if you decide to go this route.
I was going to go with the Saito 150, as its performance is what I felt was needed for this bird: the ability to turn a 15x10 or a 16x8 APC prop in the neighborhood of 8400-9000 rpm on the ground.
This calculates out to over 10 pounds of static thrust and between 75-85 mph. I ended up going with the OS 120 AX because the various RPM posts on the engine indicated its performance is almost identical to the Saito 150. The advantage of course being that it costs a little more than half what the Saito does.
One of the best things about the 120 is that it can be purchased without a muffler for $30 less than with.
http://www3.towerhobbies.com/cgi-bin...&I=LXLGH9&P=ML
That's great because the standard OS "power box" muffler is one ugly sob, [:'(] and it weighs a lot- over 7 oz.
I went with the 120, mounted inverted, and I used a Macs muffler stuck out the left side, and I used the OS aluminum engine mount.
http://www3.towerhobbies.com/cgi-bin...?&P=8&I=LXSXF2 Jtec makes a wrap around, inverted pitts muffler for the 120, but I didn't find out about that until after I had purchased the Macs. There's not a lot of difference in price between the two, so if you're going with the 120 choose your preference. I might switch over to the Jtec in the future, as it would require very little modification to my cowl- just filling in the Macs hole on the left, as I already have a big cooling outlet on the bottom near the firewall. I think the Jtec will fit in there without cutting another hole on the left side but I'm not sure on that. I do know that anything other than a factory style muffler is going to be restrictive and will cost some rpm.
The 120 and the Macs did two things I was looking for: 1. The performance I wanted 2. A fairly large weight savings over a four stroke as the OS is 8 oz less than the Saito, and the Macs muffler can't weigh more than an oz or two. (Someday I'll put it on my powder scale.) This was important, as I was able to balance the plane by putting a 4 oz, 1450 milli amp battery in the tail wheel well. No lead required.
The system won't let me upload any pictures right now, but if you go back to page 70 there are some on the 6th post down from the top.
As I found out during my first flight, this setup gives a comfortable margin. There's no need to use full throttle except for aerobatics. Easing the throttle in resulted in a very scale like take off, and I can cruise around all day at 50% power or a bit more.
Don't let the idea of an inverted two stroke scare you away- properly setup, the 120 doesn't know it's upside down. There are just a few things you have to pay attention to:
1. Use a Hayes Slimline 12oz tank
http://www3.towerhobbies.com/cgi-bin...?&I=LXK835&P=7 12oz is what OS recommends, and there's no need to use anything bigger. This tank is skinny enough that you can mount it on a 3/16 ply or balsa floor that rests on the bottom of the former openings, and the center of the tank (top/bottom) will come out exactly on the center line of the spray bar in the carb.
2. Break the engine in on a test stand mounted inverted. After you've run a tank through it at slobbering rich full throttle, lean it to a few clicks off peak, then idle it down and start adjusting the low speed screw. This is the real secret to inverted mounting- proper carb adjustment. The nice thing about the new OS carbs is you can adjust the low speed needle with the engine running, safely.
The critical thing is to use a tach and get the engine down to its specified idle speed before you start adjusting the low needle. (1800 rpm or less) If you idle it down and it loads up and quits, which it probably will, turn the screw in half a turn and try again. It will probably keep running. Then, pinch off the fuel line and wait a bit. The engine should speed up just very slightly. If it speeds up a lot, it's still too rich, if it slows down immediately or dies, it's too lean. Play with it until you're happy, then go back to wide open and do the pinch test. (If the engine wants to die or won't speed up, you closed the low speed needle too much- back out 1/4 turn.) You want the same results- a slight increase in rpms. Play with the high speed till you get it, then go back and check the idle adjustment, followed by the high speed, then you should be ok. After this process, I can idle my 120 down to 1800 for 2 or 3 minutes, smoothly open the throttle wide and it will go right back to full power with no spitting or coughing. Inverted.
The temptation with most builds is to use a tank that is too large for the engine. The exhaust system can only supply so much pressure. If the tank volume is too big, the feed pressure to the carb will not build up quickly enough after running at idle, and the engine tends to starve during acceleration. I'm convinced that this and failure to properly adjust the low speed needle are the cause of most of the dead stick landings you read about on RCU on a daily basis. With the 12oz tank, my engine will run over 20 minutes at a combination of 1/3 full, 1/3 half, and 1/3 idle speeds. I made a 7 or 8 minute maiden flight and had more than half a tank left.