Maybe you can use a mixture of vegan yogurt and cream cheese to replace the sour creme? Although you might be fine using sour creme. It's only a relatively small part of the recipe, so you probably won't taste it as much as when eating it straight up.
The recipe I looked up had four eggs, split into yokes and whites. You'll need quite a bit of egg replacer and baking powder to make up for the air in the beaten egg whites. I found this vegan version (no sour creme!). http://vegweb.com/recipes/excellent-poppy-seed-cake