[SpudRider]

This information is incorrect; I made an error. :oops:
Here's my current, best guess concerning the engine design. Your ZS172FMM engine is basically a Yamaha XT200/TW200 engine. It appears Zongshen lengthened the cylinder 5.5mm to gain the extra 34cc engine displacement. In addition, Zongshen modified the cylinder head so the exhaust port exits the right side, instead of the left side.

This is the correct information; therefore, I have edited this post.
It appears the ZS250GS is basically a Yamaha XT200/TW200 engine, with a larger bore. It seems Zongshen enlarged the cylinder bore 5.5mm in diameter to gain the extra 34cc engine displacement. Therefore, Zongshen is using a larger piston, and larger piston rings than the Yamaha motorcycles employing this engine block. Also, Zongshen enlarged the cylinder head, and relocated the exhaust port on the right side, instead of the left side.

If someone wished to mount this engine in a Yamaha, or Zongshen 200GY-2 frame, he might have success employing a Suzuki DR200SE, or similar, exhaust header.

Spud