Hi!
A rubber seal or any other seal is not helping in this case. The leakage is due to the fact that the inner crankase to crankshaft surface is worn out or made with too large tolerances...no steel or rubber seal will fix that!
Remember the rubber/steel seals on a ball bearing is made to prevent dirt getting in or holding grease in...not holding glow fuel in!
Picture shows open, C4 bearings from Swiss company WIB the size all .40-.49 engines on the markert use (28x15x7mm) except Rossi.

Regards!
Jan K
Sweden