I am writing my Master's thesis, and I need to describe and break down a complex mathematical (physics-based) model. This model is absolutely central to my thesis, and all of my work essentially revolves around it. I was about 5% done the task of understanding and summarizing the model when I came across a previously written Master's thesis that used the exact same model in the exact same manner as me, and includes an incredibly detailed, yet concise step-by-step breakdown of it. It is all that author's original work.
The breakdown of the model in this previously written thesis is approximately 20 pages long, far too long for me to quote it, even in part. There's really no way for me to interpret or explain this model other than the way this author has described it. This author was pretty clearly a genius... I feel like any information I add will be superfluous; while at the same time if I were to remove or greatly modify any of the information it would be extremely detrimental to my thesis as a whole.
I feel completely stuck. This breakdown of the model is going to account for around 1/5 of the total length of my thesis, so even if I were to be able to get permission from the author/publisher to copy the information, I'd essentially be taking 20% of my thesis directly from another source. That just doesn't feel right to me.
What should I do? I'm a mere 2 months away from my defence deadline (long story short I was forced to start this new thesis topic with less than a year left in grad school, so I'm extremely far behind where I wanted to be) and this model breakdown is the only thing I have left to finish before sending in my thesis for editing.
I'm quite literally pulling my hair out over this. Any help would be appreciated.