How simple? Google would be your friend here. Try variations of searching for "lost radio control plane finder", "gps radio control plane finder", etc., etc.

Basic would be something that emits a loud sound - some do it when power or radio signal is lost to the system, some by command from the radio, some with both options.

There are a variety of gps units that send "where" information to the TX (if capable), your phone (usually via Bluetooth), or a separate hand held device.

Many radios have a signal strength reading that tell you what strength the receiver is getting (if it is still working) - so can be used to home in on the area - I've done that a few times in a cornfield- walk in the direction of increasing strength, until you find the area with max, and look around. It has gotten me close - the hard one was that I couldn't find it until I looked up, and there it was suspended in the corn above my head and not on the ground.

Unplanned landings have various levels of problems. Some can result in "unscheduled rapid disassembly" of even very basic components - wires ripped loose, batteries disconnected, etc. So one has to take durability into account.
Those that emit sound may or may not be able to be heard from a longer distance. They may have their own power, but it is kept small for weight/cost/size savings - but then don't make the sound for very long.
So one has to make choices as to the cost, ability of the item to withstand damage either to itself or the larger system, resolution as to exactly where the model is, complexity of use, whether you have a general idea of where the crash site is, distance, etc., etc.