There are two existing open-source Battle Isle clones: <em>Advanced Strategic Command</em> and <em>Crimson Fields</em>. Since both can import maps and campaigns from Battle Isle, I imagine that the mechanics must be either very similar or identical (though I wouldn't be surprised if the AI differs a bit and is harder or easier). You could probably look at these if you can't find someone who has just dumped the computations from Battle Isle and want to be fully compatible.
Also, if you don't want to be completely compatible, but just make something in the genre, you might want to look at newer games in this genre like <em>Advance Wars</em> to see whether there are game mechanics that are good ideas that have been introduced over the years. I know, for example, that Advance Wars has "special powers" on a per-character basis to adjust the merits of particular tactics and thus change up gameplay style; this might (or might not!) be a good idea to use.
Just saw that you mentioned "Crimson Field", are you referring to the "Battle Isle" remake? If so, you know BI already ;) There's also another remake of that game called "Advanced Strategic Command (ASC)".
What's nice about BI is that it's an excellent 2-Player/1-Screen game. While one is planning the movement, the other one is planning the attacks. That were some fun session on the Amiga back then...