IR sensors come in two kinds, ones that work by interrupting the light between source and detector and ones that rely on reflection to direct the light from source to detector. Does your conveyor have any facility to use those methods?
For interrupter types there has to be a gap or opening in the belt that lets light pass through periodically, the source is one one side and the detector on the other.
For reflective types, source and detector can be on the same side but something reflective has to pass by them periodically.
If the conveyor is driven by a gear or sprocket mechanism, there may be other ways to do it that are non-invasive.
Brian.