Even if you know that particular weapon's OP cost, you still have to scroll through all the weapons with similar OP cost to find it.
Filter options when selecting a weapon mount include weapon type and damage type, as well as being organized by OP cost and size. Are there really so many different weapons that are (for instance) Large, are a Ballistic weapon, and deal Kinetic damage that it's ridiculously hard to find a particular one even after applying those filters (even if you don't vaguely remember its OP cost)? That would require a large number of mods installed, or a few that add a
lot of a specific weapon type.
It's an interesting idea, but my 2 cents is that the weapon select menu already takes up most of the screen vertically; adding a search text box would only make it even bigger or show less weapons at once. This also seems to be handled in all but the most extreme cases by the existing weapon filter options, and filtering by weapon properties rather than a specific name imo is more useful in the context of choosing the right weapon for the job: for instance, you might originally want to mount a particular weapon but then realize that another fills a similar role but costs less OP or has a better damage profile for just a few more OP.