You need to remove that blade boss, which is pressed on to a taper, so you have to use a hammer and anvil process to persuade it to pop off. Remove the engine from the base plate, and set it up with the crankshaft horizontal so there is a heavy piece of steel flat and square against the underside of the parallel part of the boss. Smack it hard with a hammer, vertically down onto the upper parallel part of the boss, directly opposite the anvil. The squarer and more rigid your setup, the easier it will be to get it off.
It looks fairly rusty - you should squirt it with Penetrene or WD40 the day before you try to get it off, allowing it to enter the joint between the boss and the crankshaft.
