I believe you will not need to change anything if you use the Itch client to install Maelstrom to C:\Maelstrom and install ERAS 2 to C:\Maelstrom\games (you will have to create the games directory first). If you do that, the sample shortcut Maelstrom ERAS 2 (in directory Itch will create for the Maelstrom engine, C:\Maelstrom\gentlemen-of-fortune-maelstrom-engine), will have a Target: C:\Maelstrom\gentlemen-of-fortune-maelstrom-engine\DirectX9_32bit\Maelstrom.exe and a Start in: C:\Maelstrom\games\gentlemen-of-fortune-historical-eras-module-2. If you want to run 64 bit, just change the shortcut Target to: C:\Maelstrom\gentlemen-of-fortune-maelstrom-engine\DirectX9_64bit\Maelstrom.exe. Double check your drive to make sure those are the valid locations where you installed everything. There is also a sample Config shortcut for ERAS (every flavor of the mods, COAS, GOF, etc. will need their own), Maelstrom ERAS 2 Config, the Target is a little different, with a path to Config.exe and a quoted path to the mod you want to configure, Target: C:\Maelstrom\gentlemen-of-fortune-maelstrom-engine\DirectX9_32bit\Config.exe "..\\..\\games\gentlemen-of-fortune-historical-eras-module-2" The double dots work if you put the games directory in the suggested spot, but will also work if you change the quoted part to the full path on your machine as Target:
C:\Maelstrom\gentlemen-of-fortune-maelstrom-engine\DirectX9_64bit\Config.exe "C:\Maelstrom\games\gentlemen-of-fortune-historical-eras-module-2."
The key is just make sure you know where Maelstrom.exe on your machine is (3 different types, DirectX8 32 bit, DirectX9 32 bit, DirectX9 64 bit) and where you put the ERAS 2 files. If you know the paths, copy/paste those paths to the Target and Start in of a shortcut and launch with that shortcut.