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The following is from a savefile. If you change the value 18 in the marked line to 34 you will change the required supply centers to win.

Code:
scenario={
name="Paul Bakken's 1898"
turn={
    year=36
    season=spring
}
picture="gfx\\menues\\default\\map_preview.tga"
description="This is the standard scenario, except that each Great Power has only one unit at the beginning of the game."
visual_start_year=1898
map="default"
rules="Tutorial"
difficulty=2
seconds_in_phase=0
long_phase_time=300
short_phase_time=60
briefing={
    name=""
    text="This an adaptation of the 1898 variant that is popular among Diplomacy enthusiasts. This scenario starts in Winter 1898. Unlike the standard scenario, each Great Power starts with only one unit located in its Capitol. National borders are the same as in the standard scenario, but you must occupy your other home supply centers before you can benefit from their supply capacity or build new units on them."
    windowType="BriefingWindow"
    textureFile="BriefingPicture"
}
win={
    name=""
    text="You defeated the opponents through superior skill and guile."
    windowType="Endgame_win"
    textureFile="WinPicture"
}
draw={
    name=""
    text="In a hard fought game you were not the winner, but you survived which is better than nothing."
    windowType="Endgame_draw"
    textureFile="DrawPicture"
}
lose={
    name=""
    text="Your superiors have unfortunately bested you. You are out of the game."
    windowType="Endgame_lose"
    textureFile="LosePicture"
}
victoryCondition={
[COLOR=Red][B]    supply_centers_for_victory=18[/B][/COLOR]
    truth={
    }
}
 
Nice. SP game ends always when you have best conquest spree going on, so this at least gives bit more lenght to it. If not much.