Depends on if you care about preserving the highlights. A Linear to sRGB gamma transform would suffice, whether it’s the super simple pow function or the more complex function that keeps the linear toe in a sRGB image.
Since you have the original scene-referred linear image you have a lot of freedom as to how you want to tonemap it if you want to preserve any details in the upper range. You could rebuild Hejl, Reinhard, Uncharted/Hable, etc. UE4 uses ACES by default which can be seen in TonemapCommon.ush in ACESOutputTransformsRGBD65 for standard sRGB displays. But if you want something simple, you can use Stephen Hill’s or Krzysztof Narkowicz luminance fits * 1.5 to match Unreal as close as possible.