YouTube is infested with videos of squids riding motorcycle wheelies for long periods of time. Hopping the front wheel a few inches of the air for a few seconds under hard acceleration is one thing, but a sustained wheelie is a different beast: in order to cruise at steady speed, the chassis has to pitch up very high, perhaps 70 degrees from horizontal, as seen in that video.
So what does this do to the engine? Is the oil pickup near the back of the sump, thus avoiding oil starvation? What about all the oil now being churned by the crankshaft? Are these engines making froth that gets spewed out of the crankcase breather and into the airbox? How about the fuel system - does gasoline find its way out of the tank vent and into the evap canister (if so equipped)?
I can’t say for road bikes, but when I was a teen I raced motocross. On long dirt roads I was able to ride a wheelie for at least a 1/4 mile and it never caused an issue with the engine. Of course when doing so the revs were kept fairly low, changing gears when needed.
How long ago was this? Motocross bikes used to be almost exclusively two-stroke, burning premix; if that’s what you had, then there’s no special concern about engine lubrication, and there certainly wasn’t an evap canister onboard to get flooded by fuel coming out of the tank vent.
Some motorcycles, especially sportbikes, are designed to handle prolonged wheelies. Cruisers are significantly less likely to be designed for it. And yes, although I personally have never done it, you can wheelie a cruiser.
Generally speaking, wet sump engines are much more likely to suffer from oil starvation in a wheelie, though many sportbikes use wet sump engines and have the oil pickups positioned in such a way that it’s not an issue. Dry sump engines (engines that use an external oil tank and pump the oil into the engine) are much less likely to have a problem.