You do not need to alter the unreal engine code for this. You can use pragmas to specify exactly what parts of the fp:precise / strict / except / fast specifications you want, for more information see float_control pragma | Microsoft Docs
I suggest you make a header with your specification and you include that one in every file where you need this. You can only enforce this by an informal policy in your project, but at least it allows you to have full control. Similar flags are available
My header looks like this, basically emulating fp:strict:
#pragma once
// This is practically equivalent to /fp:strict, see https://docs.microsoft.com/en-us/cpp/build/reference/fp-specify-floating-point-behavior?view=vs-2019
#pragma float_control(precise, on)
#pragma float_control(except, on)
#pragma fenv_access(on)
#pragma fp_contract(off)
#define _CRT_SECURE_NO_WARNINGS
As you see I also turned off fma optimizations and secure warnings.
Some more information can be found here:
For Clang and other compilers you may want to use with Unreal Engine, you should be able to find similar pragmas.