Three reasons to not use O-rings on the crankshaft.
1, An O-ring is classed as a "static" seal. If used on a rotating part it will wear quite quickly and when dirt and grit are added, the O-ring cuts into the motor housing and crankarm.
2, If the motor has been used, then there will already be grit and dirt behind the O-ring when you fit it, trapping the dirt where you don't want it.
3, An O-ring will add friction.
This is why we had quad ring seals custom made to create as little friction and wear as possible. This is also why we don't sell them outside of our upgrade, because it's damaging to seal this area without stripping and cleaning the motor thoroughly first.