One of the guys over on MEM forum which is where people who build things tend to post has a lot of interest in boat/ship engines and has recently built a model of that exact engine, all on a Sherline too.

This link should take you to the last page of the build thread where there is a video of his model. The preceeding 46pages are quite interesting too.

https://www.modelenginemaker.com/index.php?topic=12874.675

Looks like a version of the Maudsley marine engine. Seem to have been quite popular as early marine engines, presumably because of the modest pressure requirement and compact size.

The Great Western used oscillating engines too (not sure what type). On a visit, I was peering around trying to spot the (mock up) cylinders, then it dawned on me that these huge things that looked like large round water tanks were actually the cylinders (70 or so inches in diameter, 7 foot stroke, gasp!… the age of heroes!). Also supposed to operate on an almost infeasibly low pressure.

Airfix made a kit of a Maudsley paddle engine, which is actually quite good and worth getting hold of if considering a model of this type. It has inspired me to at least consider how parts could be fabricated or cast to allow a working model to be made. I doubt if getting around to even making the plastic version is likely now!