Is there an IR receiver with external IR diode?
Most IR Receivers have circuitry inside to strip the (typically) 38khz carrier wave used in IR remote applications. The Diode is closely matched to this, and is often encased in a shield to protect against noise. You would be hard pressed to find a different form factor.

What you can do, if the Lego brick only allows a typical 3 or 5mm led to be inserted, is use a piece of 3 or 5mm clear acrylic rod, cut to a short length and flame polished, inserted into the brick, with the IR receiver inside facing the rod. A cheap light pipe.