After some more testing of quite different ideas, none of which were quite right. It belatedly occurred to me to try the current version of MacDuff as it is supposed to work up to the 1880's. Yup. It worked just fine once I added back the Prone rule that seems to have gone missing and made a slight tweak to the Orders Check to make units under fire pay the penalty. I'm not sure yet if that will be a general tweak or a late 19thC one. Every thing else worked fine. Next step, sometime this weekend will probably be an Nku khu ambush of a British supply train.
Archive footage of an earlier clash between British & Nkh Khu from 2 years ago.