Sorry that I can't say who made the engine but can say that it is not a Uniflow engine, to my knowledge Stuart never made a Uniflow engine.
It looks as though it has been designed on the Stuart high speed engines, STAR, SUN and SIRIUS engines which were twin cylinder single acting piston valve engines of inside admission, all of which were driven by bevel gear and shaft to the overhead piston.
Uniflow engines have slots at the bottom of the cylinder to let the exhaust escape into a jacket at the bottom of the stroke, or in early engines the exhaust was allowed to vent into the atmosphere.
Here are some pics of a Uniflow Poppet valve high speed engine, showing the exhaust slots at the bottom of the cylinder.
Hopefully somebody may be able to throw some light on the maker.