I would say Curly's decline shows in "The Yoke's On Me", as well as "Crash Goes the Hash." Not so much physical, even though he was a bit slower in movement, but in voice. His voice in "Yoke" grates on my nerves during the "feather dance", and I'm not sure if that would've been because of a stroke, or just the voice he chose. In "Crash Goes the Hash," he seemed to use his regular voice a LOT. Maybe I'm wrong and need to watch it again, but I remember his regular voice in more scenes than when he tells Larry that the Prince and Old Lady Van Bustle were over by the piano.
Then again, it was mentioned in one of the books (I forget which one) that Shemp replaced Curly during personal appearances in the early 1940s, so maybe there's a change before 1944.