.map format, not .BSP. BSP needs to be compiled by a compiler that culls faces that won’t be visible, creates PVS and lightmaps. Either way, personally I think building main CSG layout is what I want to use either Trenchbroom or DarkRadiant (Doom 3, which I have more experience with), and everything else can be done in UE4.
I am not sure what displacement is used for in HL2 maps, but I doubt it’s something you couldn’t have done in UE4 using landscape tool more efficiently (displacement sounds something you’d use for terrain).