Look in the python script of whatever map generator you want to use, and modify it yourself. Most of the scripts use TerrainGenerator() to generate the terrain. It uses the following default values:
# class TerrainGenerator:
# "If iDesertPercent=35, then about 35% of all land will be desert. Plains is similar.
# Note that all percentages are approximate, as values have to be roughened to achieve a natural look."
# def __init__(self, iDesertPercent=32, iPlainsPercent=18,
# fSnowLatitude=0.7, fTundraLatitude=0.6,
# fGrassLatitude=0.1, fDesertBottomLatitude=0.2,
# fDesertTopLatitude=0.5, fracXExp=-1,
# fracYExp=-1, grain_amount=4):
So, if I want to cut the amount in desert in half, I call it as follows:
terraingen = TerrainGenerator(16, 18)
The 16 overrides the default of 32.
Removing the tundra and snow should be similar.