Stirling engine? Just no, they could have done so much better...
Simple Stirling engines can operate without sophisticated seals, but they do output bugger-all torque. I guess that’s why they’re using a gearbox, but it’s a bit silly. I guess the main limitation is that they can’t build up pressurised steam with their metallurgy, the blast furnace may help this. The first steam engines used vacuum pressure from cooling steam to produce force on the piston, but were rather inefficient and bulky. I’m wondering if a steam turbine is the way to go, if they’re already using a gearbox.
Also if you’re industrialising anyhow, might as well make flying shuttle looms and printing presses. I hope the author did some research into iron and coal deposits in the region.