Transmission/clutch Problem Unimog

Driving my 1962 Unimog to the beach at 60 mph, suddenly lost power could only keep 45. Pulled over and realized had lost ability to change gears. Gear oil in the transmission had been leaking. Smoke and burning smell came from transmission oil filler tube. However, the unimog continued when started to try to crawl foward, neutral was attainable only by selecting neutral in the foward/neutral/reverse lever. Unimog sat for a week, when started a week later the truck started up, shifted freely but lacked power and shuttered. Upon investigation the transmission fluid and the interior of its casing looks clean, without shards or “trash” the clutch seems worn. Question is while a warn clutch explains most of this, what caused the burning fluid smell which came primarily from the transmission? And does the clutch slipping explain all of these symptoms?

is the leak related?