the Orings on the tappet covers only keep the oil from oozing out. If one was bad there would be oil all over that side of the engine. Eventually all the oil would leak out, but that would take a long time and would be making a noticeable mess on your garage floor.
If all your oil has leaked out and you did not notice or check it, that would be very bad.
Unless you have a California bike the vent hose on the fuel tank has no function that would make the bike not run correctly.
If it has no power its probably running on only one cylinder. I would not recommend riding any bike that is not running right (ie dont ride it to the shop, dont ride it home from where it fails. Some problems can cause secondary issues if you keep riding the bike.
I think you would have to start from the beginning: check the condition of the spark plugs, do a compression test, check the timing to make sure some component has not gone wonky, and then you are into the fuel system, sensors, ecm....
Mechanical and fuel system problems are not normally intermittent... most likely its an electrical or sensor problem, maybe a fuel pump (just from the face of the symptoms).
One other thought, you had not been riding for a while. How long was the bike sitting in the garage? Did you do anything to stabilize the fuel once you realized you could not ride, or did you drain the old fuel out of the tank?
If the bike sat for months its possible you got some jello-like fuel in the filter or something got sucked into the fuel pump, or even gummed up in your injectors. Strictly from a cause and effect point of view, bike parked for several weeks with no fuel stabilizer => goopy gas.
If a bike has sat for a long time and is then run on the old gas, it can start up and run somewhat normally at first. Then as the old gas is run thru the engine it can gum things up worse and worse. Since your started riding again have you run a fuel cleaner in the gas? Someone could make a good bit of extra money selling Seafoam to people on this forum :^)
Last edited by KCW; 10-06-2018 at 06:22 AM.