That little rubber ring may not look important but every time a cylinder fires, the crankshaft accelerates and then slows, setting up a pulse of energy along its length. The flywheel end will not accelerate rapidly but the belt end will. To prevent the heavy pulley from storing that energy and forcing the crankshaft forward when it wants to relax, a rubber separator is used between to pulley core and most of its mass. The other name for this part is "vibration damper".

The short answer here is that changing to a solid pulley will very likely result in a broken crankshaft (yes, a casting failure). The B21 engine used a forged crank that could withstand the flexing caused by this secondary pulse.