You probably realize that the main verb ("struck") needs an indirect object in order to make the conversion. The sentence should read
"What struck me/you/him/her/us/them/people/everyone most was their willingness to work." Since there is no indirect object, you will need to include one.
Let's try with "everyone," since it's the most general. I am not using "them" or "people" so that they do not clash with "their," which would make the sentence confusing.
Now we have "What struck everyone most was their willingness to work."
Then, as in the Passive Voice the indirect object becomes the subject of the Passive sentence, we start with the subject we have added.
This results in "Everyone was struck most by their ability to work."
Remember that whenever you need to go from the Active to the Passive voice, the verb must be transitive; i.e. needs an object that will become the subject of the Passive Voice. In your sentence, this object is tacit, but you have to make it explicit in order to get your new sentence right.