Unfortunately if you don't have REPENTOGON the game can't lock the cursor into the game window, so you can't move the mouse too far in any direction before it hits the edge of the monitor or loses focus on the game.

That said I have made a simple macro that keeps the cursor in the middle of the screen and set up a version of the mouse look code specifically for using the macro.

I used AutoHotkey and have included the ahk file as reference in the mod, (found in either 'Documents\my games\Binding of Isaac Afterbirth+ Mods\firstperson_2015147251' or 'Steam\steamapps\common\The Binding of Isaac Rebirth\mods\firstperson_2015147251') but you can use whatever system you like, it just needs to be able to move the cursor INSTANTLY to the middle of the game window and wait a few frames before moving the cursor to the middle again on an endless loop.

The file I used uses CTRL J to start locking the cursor and CTRL K to stop, you can tweak the 'sleep' time to adjust how sensitive the mouse turning is, smaller the number the slower the turn speed.

To enable the 'macro mouse mode' just open the main.lua file, look for the 'macromouse' value near the top and change it from 'false' to 'true'.