German to English Science Poetry & Literature Heisenberg quote

Any Heisenberg fans out there this evening? I have the following excerpt from something Heisenberg wrote - as far as I can tell, it is from "Physics and Beyond" (New York: Harper and Row, 1971). My problem is that I have found much of the quote in English on the web, but a few parts are missing, like the first sentence of the following excerpt, as well as several segments later on.

So my question is this: Does anyone just happen to have this book lying around and can look up the part I need? And barring that, how is such a situation usually handled? I.e. using the original source to the extent it is available, of course, but I can't really pass off my own translation of any "gaps" as part of the accepted (published) translation. This is for a magazine article, so footnotes are not really an option. Could it be as simple as putting my "filler" in square brackets?

TIA

„Einige Tage genügten, um den am Anfang in solchen Fällen immer auftretenden mathematischen Ballast abzuwerfen und eine einfache mathematische Formulierung meiner Frage zu finden. In einigen weiteren Tagen wurde mir klar, was in einer solchen Physik, in der nur die beobachtbaren Größen eine Rolle spielen sollten, an die Stelle der [alten] Quantenbedingungen zu treten hätte. Es war auch deutlich zu spüren, daß mit dieser Zusatzbedingung ein zentraler Punkt der Theorie formuliert war, daß von da ab keine weitere Freiheit mehr blieb.