UltramasterBDJ´s self-made X-Codes/N64/Mario Party 1
Mario Party 1

title-screen modifier [NOTE #1]
850f990e:xx00

character modifier Player 1 [NOTE #2]
840ffaa4:000X

character modifier Player 2 [NOTE #2]
840ffad4:000X

character modifier Player 3 [NOTE #2]
840ffb04:000X

character modifier Player 4 [NOTE #2]
840ffb34:000X

stagemodifier [NOTE #3]
840f9dab:000x


NOTE #1:
XX - description of title screen

00 - Mario's rainbow castle
01 - Luigi's Engine Room
02 - Peach's Birthday Cake
03 - Yoshi's Tropical Island
04 - Wario's Battle Canyon
05 - DK's Jungle Adventure
06/FF - the title-screen you get when you start the game the very first time / when you beat the "Eternal Star"-stage
07 to 0B - *crash*
0c - weired shadow-animation
0d - a black square on white background
0e to 1f - *crash*
20 - text "DK's Jungle Adventure"
21 - text "Peach's Birthday Cake"
22 - text "Yoshi's Tropical Island"
23 - text "Wario's Battle Canyon"
24 - text "Luigi's Engine Room"
25 - text "Mario's rainbow castle"
26 - text "Bowser's Magma Mountain"
27 - text "Eternal Star"
28 - *crash*
29 - text "Minigame Stadium"
2a to 30 - *crash*
31 - text "lite play... turns..."
32 - text "standart play... turns..."
33 - text "full play... turns..."
34 to 4a - *crash*
4b - image of Mario's head
4c - image of Luigi's head
4d - image of Peach's head
4e - image of Yoshi's head
4f - image of Wario's head
50 - image of DK's head
51 - text "Mario"
52 - text "Luigi"
53 - text "Peach"
54 - text "Yoshi"
55 - text "Wario"
56 - text "DK"
57 to 62 - *crash*
63 to FE - ??crash?? (not checked)
FF - see 06

NOTE #2:

0 = Mario
1 = Luigi
2 = Peach
3 = Yoshi
4 = Wario
5 = DK

NOTE #3:
0 - DK's Jungle Adventure
1 - Peach's Birthday Cake
2 - Yoshi's Tropical Island
3 - Wario's Battle Canyon
4 - Luigi's Engine Room
5 - Mario's rainbow castle
6 - Bowser's Magma Mountain
7 - Eternal Star
8 - Tutorial (unfortunatelly you can't explore this stage freely, as it will play the usual tutorial as well)
9 - Minigame Stadium
A/B - ...for some weired reason this will lead you to a memory-mini-game, which freezes after completing.