If the crankshaft is rusty, it will have expanded in diameter - rust is much bulkier than the same quantity of iron or steel. When it expands into the bore of the clutch, the two parts become locked together and are not all that easy to shift. I suggest you proceed as follows.
1. Separate the two halves of the clutch, by removing the three screws around the perimeter.
2. Remove the engine, complete with attached engine-side clutch-half, from the mower.
3. Soak the crankshaft extension and the bore of the engine-side clutch-half in Penetrene for a couple of days.
4. Use a fairly large 3-jaw gear puller to push on the end of the crankshaft whilst pulling on the outside of the clutch-half. Apply a fair amount of pressure then tap the clutch-half axially with a hammer, working around the outside of it. Then tighten up the draw bar again and repeat.
If you do all of this carefully, you will not damage the clutch-half.
