Hello from Macandrew Bay.

To get at the crankshaft you have to first remove the magneto base plate,
and then the cylinder block. If you can't remove the cylinder block at
least take out the four bolts that hold it in position. It sounds as if you
have already removed the five bolts securing the two halves of the crankcase
together so all that will be stopping the crankcase from separating is any
sealant (gasket goo etc) that has been put on the faces of the joint to stop
leaks. Try gently tapping round the joint with a soft hammer or block of
wood to release it. The top half should lift off.
Hope this helps,
Jack