ORIGINAL: bgw45
The closest engine that "might" fit your needs could be a Syssa 1.80. It has rear exhaust. I don't thing turning the jug on a 20 dle is possilbe as there are porting issues through a cut-out piston skirt.
Cut-out on the piston skirt? I didn't see a cut out when I rotated the piston to TDC and exhaust porting is controlled by the heigth/width/location of the port in the cylinder. The pistion shouldn't allow the exhaust port to be open to the case or your intake charge would escape through the exhaust port.
edit: just found Jody's pics of the internals in post#625 and I see the cases isn't symetrical so the answer is no.
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