If you're just after the reactions you'll want to start with all the reaction_X files. Then copy over most of the item_X files to avoid missing the tools, added weapons, gambesons etc that these recipes allow crafting. I'll list which are essential below.
Then you'll want material_template_default and tissue_template_default, as these add the material properties allowing you to salvage cloth/leather, dry chitin, make gut thread, skull helms, etc.
Note that doing this and not copying over the creature files will effectively buff every mundane critter in the game to use megabeast-tier body materials, because of the way I tricked the hardcoded generation of demons, titans, etc to use upgraded materials.
The easiest way to fix THAT would be to also copy over b_detail_plan_default, but you're going to want to edit it a small bit. Look at the second body_detail_plan listed in the file, titled normal_standard_materials. And look at the fourth body_detail_plan, normal_chitin_materials. Just delete the "normal_" in the names of the second and fourth BDPs, then delete the first and third BDPs.
The end result is megabeasts and such are no stronger than normal animals, but no creature-copying is required.
Finally, if you want hardtack to work, you'll want to copy over plant_adventurecraft. Yep, it's a dirty hack.
Adding entity_default will port over some world flavor stuff like civs having slings, the Resurfaced civ, etc but isn't vital.
The resulting list of files copied is:
* b_detail_plan_default.txt (this gets edited)
* entity_default.txt (optional)
* item_ammo.txt
* item_armor.txt
* item_instrument_adventurecraft.txt
* item_tool.txt
* item_weapon.txt
* material_template_default.txt
* plant_adventurecraft.txt
* reaction_adv_carpenter.txt
* reaction_adventurecraft.txt
* reaction_instrument_generic.txt (only required if also copying entity_default.txt, not needed otherwise)
* reaction_other.txt
* reaction_smithing_adventurecraft.txt
* tissue_template_default.txt
If you want the changes to other armor and items, you can copy over the following item files as well:
* item_helm.txt
* item_pants.txt
If you want to retain the improvement to stones (chert, diorite, etc), outsiders starting with crude stone weapons, and the ability to pick otherwise-unobtainable plants, you can also copy over their files. These are:
* inorganic_other.txt
* inorganic_stone_layer.txt
* inorganic_stone_mineral.txt
* plant_crops.txt
* plant_crops.txt
* plant_garden.txt
* plant_standard.txt