Here are some cool DKC music mash-ups. They have a K. Rool theme in both of the Guitar Hero ones, but the piano and Phil Collins are good too, just no K. Rool theme.
Guitar Hero: Kong of Rock (DKC1 mash-up)
Guitar Hero: Rocking Kremlings (DKC2 mash-up)
Ultimate DKC medley (DCK1/2 piano mash-up)
In The Air Tonight: Phil Collins DKC2 remix
And a good song I think would fit in a Sm4sh level before you fight K. Rool:
Flight of the Zinger
As for a stage idea:
The Flying Krock
It would be exactly how the map view of it is, rather than the inside of the ship itself in the boss battle. However the inside will be included too. Just image the above picture, except in Sm4sh you are looking at the side of the ship, with the tower in the bottom left corner of the map and stickerbush in the right corner. The map itself would be a large sized map, smaller than Temple but about the same size as Bridge of Eldin.
Stage element 1: K. Rool's Keep
In the bottom left corner would be the top of K. Rool's Keep. The top of the keep is flat, and has small walls that you can use to pin enemies too and have a cage-like fight as long as you dont send foes flying. The walls are not grabbable edges however. The grabbable edges are on the 2 spires that are on the side of the keep. The spire roofs would not be as steep as the picture shows, but just good enough to stand on. This makes recovering easier, as edgeguarding would he hard to do on such a small tilted surface.
Stage element 2: Stickerbush
On the bottom right corner would be stickerbush. Stickerbush acts as a spike stage hazard, and it is impossble to land on. If used correctly however, it can save you from falling down the screen, with the cost of some damage. The escalation the stickerbush would be angled at around 40 degrees, but it will also be randomly going up and down, twisting around, so its not a flat surface either. However in Sm4sh, a refernce to stickerbush levels would be that in the center of the stickerbush would be wood planks that you can land on safely. There are 2 of these surfaces, one to the lower left and one to the higher right. The plank's surfare would be flat, but about the same surface size as the keep top. There are no grabbing edges anywhere on the stickerbush, so be careful.
Debatable extra stickerbush element: Screech
Since stickerbush is dangerous as it is, this might be too much. But for the fun of it, maybe Screech, a bird you haft to race right before you fight K. Rool, will occasionaly fly around the perimeter of the stickerbush, which would knock you back if it hit you.
Stage element 3: Rope ladder
In between these first two elements would be the ladder. However the ladder would not be leading to the keep, or attached to the cockpit as it may seem. Instead, the ladder would be directly center stage, heading down to the bottom of the screen but not reaching the bottom. You can jump to this ladder from the keep or stickerbush eaisly, and should not require using your UpB special. The ladder leads to the center of the Flying Krock and into the room where you fought K. Rool. The top of the ladder would act as a platform you can pass though, so you dont fall out the room.
Stage element 4: The Flying Krock
The flying Krock acts as where you will be doing most of your fighting. It takes up almost the whole upper center map, but not expanding past the keep's edge or the 2nd plank plaform in the stickerbush. You can stand on the surface of the Flying Krock, which is the exact shape as you see, both edges can be grabbed, no need to explain. The propellers will be in the foreground and background, and will not be a hazard. The glass roof you see will not be there. It will be present, but opened up to the battle room where you fought K. Rool. Also, the Flying Krock moves around slightly during battle, but nothing extreme. It tilts just a tiny, little bit and moves around up, down, left and right slightly. This also effects the ladder's position too, but it will still always be in reach to the lower levels.
Stage element 5: Boss battle room
When you are inside of the room, that portion of the ship's front wall will become transparent so you can see, like Luigi's mansion. It acts as another good cage fighting area, and the walls are much bigger than the keeps. The walls would be the door to the back room, and the captain's seat, so no one would be pinned into the cockpit unable to escape. The portion of the floor leading to the ladder would be 50% transparent and have a small square hole that still acts as a platform, so you could see where you can exit to the ladder.
But yeah, US then? Be safe though man, and don't mess with other stuff! Green = Good