The quickest solution would be to find as many different recordings of this piece that you can find, to listen to the cadenzas that different performers play, and to construct a written cadenza that feels similar in style. That is always what I have done as a solo singer.
Try to find recordings that say that they were made "on period instruments" or "in historically-informed performance". Those have been made by musical groups that take care to play in the style that Mozart himself employed, and to play replicas of the same design of instruments that Mozart's own orchestras used.
Consider that your modern horn is very different than the horn used in Mozart's day; specifically, the horn in Mozart's time was much more restricted in the notes it could play; it could not play all 12 notes in the octave. I do not understand the mechanics of horns, but the idea is that you might decide that your cadenza should not use any of the pitches that were not available on the horns from Mozart's time. But this is your artistic decision to make.
So do some Google searching and find out about the kind of horn or horns used in Mozart's era in Austria, and how they were constructed, and specifically what pitches and what range they could play in. Then consider applying these parameters to your cadenza.