The knock sensor is tuned based on the bore diameter of the engine. Thus, pick the engine with the bore diameter closest to yours.

Untuned knock sensors are also out there, and will require a knock filter tuned to the bore diameter of your engine. Basically, knock is identical to a musical note, and as such has a specific pitch. Pitch is based on the diameter of the cylinder, so we can cut out a LOT of false knock signals by only listening for the pitch that would be generated by a detonation event that causes the cylinder to ring.

The location on the block, high/low/left side/right side/front/rear is mostly to avoid sounds that make it through the filter or are in the same range as the desired signal, or just loud sources that overwhelm the filter totally.