Something has to be really wrong if a front intake engine leaks fuel from front. There is no spesific force that pumps fuel through front bearing. Quite opposite; it's often necessary to broach a channel from carburettor area to near front bearing to get some lubrication to front bearing. It's a sign of a healthy setup if you see a very faint ring of oil inside the engine compartment after a few flights, flown from front bearing.
Personally I see no point in sealed front bearing, unless you crash a lot in soft soil.
In rear intake engines it's a little more tricky to acchieve a controlled slow leak to provide lubrication for front bearing. After some experimenting I have settled to a 5mm long sealing area right after rear bearing, with 0,04mm clearance. Front of that it opens a little to allow shaft deformation without contact with case. That seems to be safe but just in case I squirt some oil to front bearing before a flying session, when I remember.
But mileage may vary, I only use WIB bearings with their (PEEK?) or my own made Tufnol retainer rings. L