Instrument cluster, if it has a tachometer and the maximum revs are different (which is highly likely if one is comparing a diesel to a gasoline engine). Instrument cluster again, given that the diesel engine will need a glow plug warning lamp and possibly a DPF warning lamp, and the gasoline engine won't need that. The gasoline engine will need an evaporative control system (charcoal canister) plus all the valves and hoses to make it work; the diesel engine won't. The transmission, if the engines operate in different RPM ranges, which is usually the case between diesel and gasoline. The transmission again and the clutch, if the diesel engine has a torque rating beyond what the transmission for the gasoline engine will handle and it thus needs a "beefier" transmission and clutch. The diesel engine with modern emission control systems likely requires a "diesel exhaust fluid" storage tank, heating system (for below-freezing temperature conditions), dosing pump, and a means of filling that tank. The diesel engine will require additional high-capacity electrical systems for operating its glow plugs.
Engine mounts. Subframe, if the engine mounts are in different places. Sound insulation around the engine compartment (diesels often have more of it). Front suspension springs and dampers, if the engine is of significantly different weight.