Can anyone send me some general dimensions for that cowl ?

From the pictures, it looks like the turbo-jett muffler might fit very well. Side mount the engine 90deg.... let the cylinder head sick out the one side. The muffler would likely be totally enclosed in the cowl.

For the OS 61 or 91fx the turbo 10cc version will work great. Its about 3" in diameter and stands off about 2" total from the exhaust flange (excluding the exhaust tube.

This is an image of a turbojett installed in the front of my VQ-P-51. That is a jett 46 installed with a 40 size turbo. Fits 99% inside of the cowl. I opened the chin "inlet" to allow some muffler cooling. Cylinder head sticks out the side.

It looks like the PT cowl is fairly large. take a look at the jett web site for the actual dimensions.

Bob