Modded trinket support (example mod 2)

local Mod = RegisterMod("yourModName", 1)
local game = Game()

local moddedCos = {		-- keeps track which trinket is in the player posession
	YourTrinket = false
}

-- trinkets
TrinketType.TRINKET_YOUR_TRINKET = Isaac.GetTrinketIdByName("Your Trinket")
-- costumes
Mod.COSTUME_OF_YOUR_TRINKET = Isaac.GetCostumeIdByPath("gfx/characters/yourtrinketscostume.anm2")

-- change if the costume gets applied on pickup or by smelting
function Mod:setTrinketCostume(consol,para)
	if consol == "onPickup" then		-- global. Regulates when a trinket costume is applied
		if moddedCos.YourTrinket == true  then
			if onPickup == true then 	-- check if the player holds trinkets since their costumes need to be removed
				for m = 0, (game:GetNumPlayers() - 1) do
					local player = Isaac.GetPlayer(m)
					local playerData = player:GetData()
					if ctc:GetHeldTrinkets (player, TrinketType.TRINKET_YOUR_TRINKET) then	-- check if the player holds the trinket in one of the..
														-- trinket slots (returns true or false)
						player:TryRemoveNullCostume(Mod.COSTUME_OF_YOUR_TRINKET)	-- remove costume
						moddedCos.YourTrinket = false
					end
				end
			end
		end
	end
end
Mod:AddCallback(ModCallbacks.MC_EXECUTE_CMD, Mod.setTrinketCostume)

function Mod:onNewRun(bool)
	-- reset at the begin of a new run
	if bool == false	-- is false on the start of a complete new game
	and trinketCostumesMod then
		moddedCos.YourTrinket = false
	end
end
Mod:AddCallback(ModCallbacks.MC_POST_GAME_STARTED, Mod.onNewRun)

function Mod:onModdedCostumeUpdate(_)

	-- check if trinket costume mod is active
	if trinketCostumesMod then
		for i = 0, (game:GetNumPlayers() - 1) do
			local player = Isaac.GetPlayer(i)

			if player:HasTrinket(TrinketType.TRINKET_YOUR_TRINKET)	 -- still have to check if the player has the trinket
			and moddedCos.YourTrinket == false then
				-- add the costume
				if onPickup == false then 		-- global variable which decides when the costume gets applied
					 if ctc:TrinketIsSmelted(player, TrinketType.TRINKET_YOUR_TRINKET) then     -- specific function which checks if the to evaluated trinket is smelted
											     -- returns 'true' or 'false'
						-- costume would get applied by smelting the trinket
						player:AddNullCostume(Mod.COSTUME_OF_YOUR_TRINKET)

						-- set moddedCos. to true
						moddedCos.YourTrinket = true
					end
				else
	 				if player:HasTrinket(TrinketType.TRINKET_YOUR_TRINKET) then
						-- costume would get applied by picking up the trinket
						player:AddNullCostume(Mod.COSTUME_OF_YOUR_TRINKET)

						-- set moddedCos. to true
						moddedCos.YourTrinket = true
					end
				end
			end
			if (not player:HasTrinket(TrinketType.TRINKET_YOUR_TRINKET))
			and moddedCos.YourTrinket == true then				-- check if the costume needs to be removed
				-- remove the costume
				ctc:RemoveTrinketCostume(player, TrinketType.TRINKET_YOUR_TRINKET, Mod.COSTUME_OF_YOUR_TRINKET)
				moddedCos.YourTrinket = false
			end
		end
	end
end
Mod:AddCallback(ModCallbacks.MC_POST_UPDATE, Mod.onModdedCostumeUpdate)