Awesome super sweet bike!!!! my first... anywho, getting into wheelies now. noticed when bike is struggling most during my hard "learner wheelies" will feel as though chain is missing teeth on sprocket. chain n sprocket all good, has to be internal, was using chain/sprocket tooth skip as a feeling reference.
thinking it may be rounded tranny gears due to hard/missuse/learner riding by previous owner(s)

 

Is it popping out of gear?
How are you doing your wheelies? Clutch or Power?

 

Sum up how you pull a wheelie, as precisely as you can. What gear, revs, etc. And regardless if it isn't a second gear wheelie problem caused by a partial engagement which normally ends up in simple shift into neutral front of bike slamming down and engine redlining.. you are likely going to have to pull it apart or replace it entirely. You could have a misadjusted chain or severely worn sprocket / chain that is slipping during your activity likewise with the internal portion of your front sprocket and splines.

 

both sprockets are relatively new swell as chain. stock front +2 rear. happens in both 1 and 2 gears power and clutch. power in first doing bout 30mph off throttle then full on; sound heard right before beginning of bp. its hard for me to get even close to bp in 2nd clutch ups; done at 40-45mph tac at 6 when i pull then rev to around 10 then drop clutch

 

You are going to need a surgeon to open things up and a dentist to replace the teeth that are broken. Sounds like you have teeth missing off of the cassettes themselves and or possibly countershaft issue. Depending on the miles etc, it may be cheaper and quicker to replace the engine and fiddle with the damaged one over winter.

 

sounds like it could also be a clutch thats dying. if you search, theres a few videos of mv augusta F4 launching hard at a drag strip, and he might be having the same problem you are.