Yea i agree with flathead. This definatly looks like the 31 cc ryobi with the small dia case. It has the step in the case that is a dead givaway for the small dia. I bet if you take the backplate off the end of the plate will be flat with the reed valve resessed into the backplate. I have just recieved one of these engines and have yet to do some testing on this engine But i see no reason this houldnt be a good one.

The engine mount apears homemade

I am really interested in the muffler as i have never seen one like this. Is there any markings on it that might indicate who made it? can you get some more pics of it on here ?