Coffee spills are the worst kind of helpful. Happened to me last year with a site plan. Let the paper dry completely before you try to erase the stain, or you'll tear it. A hairdryer on low works if you're in a rush. Maybe do a quick trace of the corrected area on a fresh sheet before the old one gets too brittle? What are you using to fix the line work, ink or pencil?
Man, that hairdryer tip is a lifesaver for sure. Been there with a coffee disaster on some vellum, totally ruined a whole afternoon's work. Tracing it early is smart, paper gets weird and crinkly once it dries. Hope your fix goes okay, those site plans are no joke to redo.