Tom was surprised to find Mary sitting all alone in the chapel.
Where does the missing “sitting” belong - here?
Tom war überrascht, Mary sitzend ganz allein in der Kapelle zu finden.
No, not there as a present participle but perhaps here as an infinitive?
Tom war überrascht, Mary ganz allein in der Kapelle sitzen zu finden.
In the original german sentence, “in der Kapelle zu finden” is a unit, i.e. “in der Kapelle” is the “where” part that modifies “finden”.
If you want to include “sitzend”, you can put it after “Kapelle”:
Tom war überrascht, Mary (ganz allein in der Kapelle sitzend) zu finden.
Here, “sitzend” becomes the main aspect, and “ganz allein” and “in der Kapelle” become modifiers of it, i.e. the how and where of “sitzen”.
Or you put “sitzend” after “ganz allein”:
Tom war überrascht, Mary (ganz allein sitzend) in der Kapelle zu finden.
This leaves “in der Kapelle” modifying “finden”, while “ganz allein sitzend” is its own unit describing Mary.
But using “sitzend” either way like this puts an awkward emphasis on the sitting aspect, when it should be on the location instead. I think that’s why the translator omitted this. While german can use “Partizip Präsens” (-end forms), it is not used as often as in english, and as I said it puts some emphasis on the “how”.
The english language uses this pattern quite often:
to find (someone doing X) - where “doing X” is “sitting all alone in the chapel” in this case
In german we don’t use this construction, so trying to translate it literally by using the Partizip leads to this awkwardness.
Maybe something like this sounds more natural:
Tom war überrascht, als er Mary fand, wie sie ganz allein in der Kapelle saß.
This allows to include the sitting aspect in a natural way without it being too prominent.
The original sentence is ok, too. It’s just not easy to include “sitzen” there without making it awkward or without rephrasing it.