Okay.. here is what I found. Not sure if it's ryobi (this is in a 31 cc snapper from this past season that went on clearance at Wally World)

Muffler and carb are on rear. The carb is mounted to the back plate of the engine crank case, however... this cover (backplate) is very large and would have to be cut down to be flush with the crankcase itself.

Muffler doesn't use screws to hold it on. It is held on with a spring on either side of the cylinder head which clamps the muffler to the head. Exhaust port itself is recessed into the head. Muffler has a tube extenstion which presses into this exhaust port.

The front of the crank case is wide and would have to be cut down to look correct.

People mention the Ryobi engines now have a double ring... where are both rings... both at the top of the cylinder? I only see 1 ring at the top of the cylinder through the exhaust port. No other ring is visible.

Bolt pattern at the back of the crankcase is different (mounting area) than the square 4 bolt patern of ryobi. Also, the cylinder head looks different and attaches differently.

This must be some other animal.

It appears there is another current engine in the Ryobi configuration that is also 31 cc. Original clues pointed to ryobi, but this does not look to be.

-Steve