Maybe a very stupid question, but why would you want to go past 30 in 2nd gear? My bike doesn't like it when I do that so I change to 3rd gear around 25-30.
Good luck taking it apart (more important, putting it back together), make sure you don't end up with one bolt left.

It seems you like to use a sledge hammer where a tack hammer will do.

Unless the needs it, save the overhaul for later.

Isolate the problem and fix it.

I agree with Rob, check the cam chain.  It could be fixed with only a couple of hours work.

Almost sure the cam chain adjuster popped out and the cam chain jumped a tooth.
  Pull the right side eng. cover and verify. After that decide how far do You want to go with the repairs and get the necessery parts. If You don't want to replace the chain and the slider-tensioner, just get the a gasket and a new adjuster assy and do the modification to the part that goes out. This way You will buy Yourself another 10-12 K miles cheap.

Well, if you want some kind of ego/psychological charge, tear into it. If you want to ride, then fix it & go down the road. It had No heart & will no appreciate your care. It will run & run if you make the cam open the valves at the right time for the piston to be all happy & spark at the right time. No need to go thru that engine with 20,000 miles on it unless someone never changed the oil. Make it run right & the engine is fine. Sounds like a cam issue. Check compression. If it's not okay, make sure it's not a cam timing issue causiing it. Odds are, rings are fine, bottom end is fine, piston & cylinder, fine, tranny, fine, everything is in great, serviceable condition, except a cam chain & timing issue. You can have that bike going down the road as well for $150.00 as you can for $450.00. All you are gonna do is waste a lot of time & money & change nothing about the bike.

I am trying to keep you from making a mistake, please, don't be offended, just realize, to need a rebuild with 20,000 miles is unheard of on this engine. I have seen one with almost 90,000 on the clock. Wasnt pretty, but it was running! All yours needs is a couple of days spent on it getting the cam chain & tensioner right & run the valves, It will run good again. Maybe a carb adjust, but no rebuild needed. Betchya!

So,......I guess the phrase "keep it simple" should be visited....

First,  did you check the compression?  If it's good, there is no need to pop the piston & jug.  If it's good and you pop them anyway, it's automatically new gaskets, rings and a hone job, the latter of which can be screwed up if not done correctly and then it would be a bore job, oversized piston & rings (yep, more rings...).

I agree with the rest of the group,  pop the clutch cover and scope it out .  

As for getting a modified cam chian tensioner verses a new cam chain.......your decision,  I went with the chain ($86).   (although I still think it's a only a so-so design)

Man, that's such good news. Now, you will have a ton more $$$ for gas & time to spend riding instead of doing a mechanical exercise.Have fun..