Pretty much larger tires and a front sprocket that has more teeth than the stock one is about all you can do, but the heavier tires and larger sprocket will make you lose bottom end. You will have to sacrifice one for the other. And you will probably need a longer chain for the sprocket also. And probably a larger case saver for the sprocket. I think a header with a stock motor will actually make you lose even more bottom end but I might be wrong.

 

You need to be more realistic with your suspected power gains. All those mods combined arent going to give you 4HP.

The revbox is completely unnecessary with your package because even the mildly modded EXs stop making power well before you reach the stock limiter. Power is falling off the entire time when you get that far with stock internal engine parts regardless of the pipe/filter combo.

Regarding the tires, those swampwitches you have now are robbing you of alot of power but it's probably best you stick with the 12T as long as you are running these tires. If you go with the 20s and still use the 12T, the revbox then might be a "have to". Still will not boost power would let you rev out on the top end for a few more mph.
With everything you have mentioned on what you want to change, you need to start by knowing exactly which of these tire combos you mentioned,..that you will primarily use. Then plan the rest for that setup because a 2" taller tire, that is also heavier is going to have a big impact on what you need to do to keep the RPMs where you need them,..and to keep your topend speed.