I agree with pastry mama, that Karo syrup can make an effective substitute for molasses.
However, Karo is sweeter than molasses and you may have to make an adjustment for that. On the other hand, cocoa powder can use the extra sweetness compared to "spice." What I'm saying is don't expect perfect results the first time you try these major changes. You've got a texture you like, and are trying to engineer a chewy chocolate cookie from a chewy molasses spice cookie by swapping out the molasses and spice for Karo and cocoa. Even if you get the sweetness level right, cocoa powder does things to texture that "spice" does not. Getting the sweetness, flavor and texture right with those changes is asking a lot.
You could nail it next time, but maybe not. If you don't, just keep plugging away. If your results are way off, see if you can't search for a chewy chocolate cookie. There are millions of recipes on the 'net and nine or ten of them are actually good. I suggest searching "brownie cookies," or "fudge cookies," or similar terms. In fact, if you don't like developing your own recipes but are looking for quick success, I think that may be a better place to start than reconstructing those spice cookies.
I wish I could give you more technical help, or a proven recipe. Unfortunately, cookies are just one more thing for which I have neither aptitude nor experience. On the other hand, I 'm a champ at trial and error.
Just here for the moral support. Good luck,