...Gasoline also comes out of the vent line.....
Stop what you are doing: ^THAT is the problem.
One of your float bowls is stuck open and it is flooding your engine with gasoline.
Since this bike has a fuel pump Im pretty sure the fuel pump is blocking the gas from flowing whenever you have the petcock on, but once you turn the key on , the fuel pump comes on,
and since the float bowl is stuck its flooding gas out the vent tube, and it sounds like its also pouring it into your intake valves!
This fills up the top of the piston with gas, and when it has filled up enough the piston cannot move up all the way - gas does not compress, and its slamming into your head
its called hydro-locking the piston. the same thing would happen if you took out your spark plugs and filled the pistons with oil, put the plugs back in, then tried to crank over the engine.
1. You need to free up the float bowls so gas is no longer streaming out the over flow tube. You should be able to figure out which one is stuck by pulling the tubes off the carbs, and see which one is flooding out. You might be able to run some Seafoam mixed with gas into the carb, and then tap the carb bowl with a screw driver and get it to loosen up
BUT DONT CRANK THE ENGINE until you have fixed that. You can see if the float bowls are stuck just by turning the key on for a few second to run the fuel pump, and check the over flow tubes. You do not have to crank the engine for the fuel pump to push gas into the float bowls.
2. your oil IS full of gasoline - you need to drain the oil, remove the oil filter, and do a complete oil change. A little bit of gasoline in the oil seriously compromises its properties. Dont crank or start the engine with all that gas in your oil.
I dont think you have destroyed the engine, unless you kept doing this over and over.
Do 1 & 2 and then the bike should start up and run.
Look around the top of the pistons and make sure the head gaskets have not been blown thru from the hydro-lock pressure on them.
You have a very good chance that is all you need to do.