This is a Lunar IPS patch that allows you to access all known unused tracks in Diddy Kong Racing.
You must apply the patch to a  USA VERSION 1.0 ROM.

The unused levels can all be accessed from the tracks menu.
The only exception is the unused snow mountain level, as it requires you to use the level ID of the Trickytops boss from Dino Domain to *fully* access it.

To get around this, you can use this code, which will allow you to access the level slot of Trickytops in the track selection. You can also go to Dino Domain's boss race without the code to access the snow mountain.
800DF4C7 0026
I also swapped the level model on the Tricktops rematch course. I left the skybox as-is there, so it's still a sunset.

Horseshoe Gulch can be accessed with this code, or by going to the Whale Bay level.
800DF4C7 0010

Sequence Area, which is the intro cutscene with an early hub model, can be accessed with this code, or by going to Boulder Canyon.
800DF4C7 0024

The other unused levels can be accessed from the four main Dino Domain tracks.

If you do not have access to all of the levels, you can use this code:
50000904 0000
101FCAF7 00FF
E8731E71 594F
E8731E79 594F
50000404 0000
101FCB2F 00FF
E8731EA1 594F
50000804 0000
101FCB53 00FF

This code will force the plane to be selected in the tracks menu, in case you are not able to select it.
801269C0 0002

*Side Notes*
Keep in mind that these levels, for the most part, did not have any assets outside of the track models. I did give the model-only levels skyboxes and music, to make them a tad less dull, but did not add 
The unused volcano area has a balloon in mid air because I used the setup from Smokey the Dragon's boss fight. This was the only way I could get the player to spawn in a position where they are not trapped. (Using the empty setup from ID 000A didn't work, if I remember correctly).
The waterfalls in the lagoon level do function if you use the setup files from the hub world/Central Area, but I decided to use the empty setup, 000A.

I have included a way to access the RC Car Sequence stage via the Everfrost Peak level slot.
This reveals that all of the track models seen in the title screen are on the same level model, just spread out very far! Therefore, Horseshoe Gulch DOES NOT appear in the title screen sequence!
Make sure you fly AS SOON as you enter this level, so you don't get stuck. You'll have to fly around the far sides of the map to start finding the other areas.

As for why you absolutely have to use the Trickytops stage slot to access the unused snow mountain level, I think you will understand once you play it.
The Trickytops stage slot is the only track in the entire game with a ceiling high enough so that you will not get blocked off from the rest of the level.
The snow mountain matches up almost perfectly with the Tricktops stage, to the point where the finish line at the top actually functions and the boss will reach it too.
For some reason, though, the only boss that can race with you here is Smokey the Dragon. Not even Rocket Wizpig can fly with you, as he isn't able to take off fast enough.
Just follow Smokey, and you can fully race through the snow moutain! Be warned, though, if you try to cheat and just fly to the top, the track might not recognize your completion!

Ceiling heights seem to be assigned to the level IDs. So even if you change the stage setup (the objects/markers), the ceiling height will not change.
Perhaps the ceiling heights could be modified, but I don't know of a way to do it. The Goldeneye Setup Editor does not allow you to change them, so a hex edit or gameshark code would be needed.

The patched ROM and unused levels are confirmed to work on the EverDrive64!