Hi,Martin. The first thing to do is keep moisture out of your crawl and that will keep the source of the odors away. Lay down 6+ mil polyethylene sheeting on the dirt and bring in some gravel to keep it in place. Be sure that the water off your roof from downspouts runs at least 6 feet away from your house and that the ground slopes away from your house for the same distance.
Natural air circulation is better that the fan, if you can arrange it. Two vents, one on a north or west wall and one on a south or east wall will keep air flowing. You should have one vent per 250 sq. ft. as a minimum also. You should have insulation between the floor joists for best performance in cold weather, otherwise, block the vents in the winter. For now, have the fan blowing out.
To get rid of any odor that is there now, first look for black mildew patches on your underfloor wood. If you find it, wash it with one cup of household bleach to one gallon of water. If you can't find any, open 2 to 4 boxes of baking soda and leave them on the floor of the crawl. These will absorb nasty odors just like they do in your refrigerator. Replace them with new ones about once a month.
Don't forget the most important thing--get the poly down to keep the moisture out.