Chances are you will need a special spanner hold the flywheel while you tension up the flywheel nut on reassembly. Don't underestimate the importance of the flywheel being tight enough on its taper to not move on the crankshaft at all. Clean the taper surfaces with alchohol & do not oil them.
If it comes loose you are lucky if it shears off the key in one go, messes up the timing & stops the engine.
If you are unlucky it flogs back & forth before it shears the key so much that it buggers up the keyways in the flywheel & crankshaft. Big job to fix.