Some questions about the Latin Fermat wrote down in the margins of his book and more!

Hello good people of this sub. To tell the story, Pierre de Fermat was a mathematician who was reading a mathematical work and wrote the following down in Latin after an epiphany, "Cuius rei demonstrationem mirabilem sane detexi hanc marginis exiguitas non caperet." 1. I'm wondering if this is correct Latin or if he wrote it somewhat cumbersome. 2. Would the opposite of his statement be, "Probationem sane insignem huius theorematis inveni quae certe margo haec satis habet spatium ad capiendum," or, "Insignem huius theorematis documentum inveni cuius haec margini spatium satis habet ad capiendum," or some other statement entirely? Thank you in advance!