How can I re-use someone's mathematics thesis without plagiarizing?
I assume that someone's PhD thesis is a larger piece of work than a section of your report. Therefore, I don't see how you can avoid summarizing the thesis to fit your work. And while you are at it, you get the chance to deepen your understanding of the subject, because you will basically be writing a review of this someone's thesis in light of you report. That is a prospect you shouldn't miss, as in academia as well as in industry, you will be expected at some point of your career to "digest" some work and produce some implication of it.

Citing the thesis is a must, if it is used as a reference, there is no debating it. I would recommend writing your section "your way", but not just changing some details to mask your copy-paste trail. Put the material in the context of your report. You don't need a clear mind and conscience to use something you learned from a source. If it "looks like" the said thesis, that is not a problem, as long as it fits and you have gained significant understanding from it.

PS: If you did a simply copy-paste, you would likely find that the text still needs some polishing to fit your report. That can amount to a significant amount of work.