i've got some for barrows, but only taken off my engine.you could probably make the boiler but I doubt you could make the motion, but you could just copy it!
I like Bobs idea of making a fowler, i'm not sure any survive.

Having looked through the register, not sure if many or any Wallis survive? But I fear someone will prove me wrong. I even thought of a Taskers as only Two survive, and all ere slightly different.

Or an Aveling or a Burrell , single crank of course !

Why not a little Tuxford? You could most likely fabricate the majority of the components. Very nice little engines. But the Fowler idea sounds good...