Bobby,
I have one of the in cowl mufflers and have tried to use it with a Super Tigre S-90 unsuccessfully (works greatly with 1.20 4cycles). All the clubs I belong to require engines to be below 96 or 94 db which is accomplished by over sizing the prop and adding baffles to the mufflers. Now that I have set the tone getting the back pressure from the muffler and the extra mass of the prop's mass balanced well enough to allow the engine to run. When I put the in cowl muffler on the 90 the engine acted like it had too much back pressure from the muffler. As the back pressure builds the pressure in the fuel tank builds increasing the amount of fuel through the carb. And the engine eventually goes rich and dies. Unhooking the muffler pressure will solve that problem but cause new ones. The easiest way I've found to elevate the excess back pressure was to drill 1/8 in holes in the muffler until the engine runs right. Of course the engine will get louder, and if you don't know what you are doing you could ruin the muffler. How about mounting the engine at 90 degrees plus 15 0r 30 degrees so that you can run the swinge muffler in the cowl and fuse. Block out the fuse around the muffler.

For fueling I use two formost bulkhead fittings screwed into the cowl. One fitting goes to the tank the other to the carb and I hook the two together with a small peace of fuel tubing to run the engine. The advantage is that all the parts are simple, inspect able, and field replaceable.

Joe