Platz are about as standard Mennonite as possible - they come in just about every flavor of tree fruit. That being said, use a tart seasonal fruit preferably - rhubarb, cherries (sour preferably), Italian or Damson plum and apricot.
In regards to fruit choices - sour cherries should be prepared like a pie filing - in that they should be thickened and sweetened prior to use. Fresh or frozen sour cherries are ideal for this but they are hard to come by - you could try using canned in juice - though their flavor is weak.
2 cups all purpose flour - you could use 1/2 whole wheat pastry flour
3 tsp baking powder
4 T sugar
1 tsp salt
2T shortening - not just EB - you could try to use EB's margarine but the crust will be different in texture and you will need to reduce the salt according
1 cup soy creamer
1 egg replacer
Fruit of choice 4-6 cups of fruit chopped - rhubarb was in 1cm cubes
Cinnamon - not for the cherries but good with the other fruits
3/4 c flour
1/2 tsp baking powder
2T EB butter - cold
1T soy creamer with 1 tsp vanilla
Sift the dry ingredients in the crust, add the shortening until combined into the flour in small "peas," add the liquid and press out on to a baking sheet/cookie sheet. It doesn't look like it will reach, but it will, be patient.
Cover evenly with fruit, sugar and cinnamon. Set aside. Combine all the ingredients in the crumble minus the creamer, making sure to evenly distribute the EB, add the creamer slowly - you will likely not need the entire 1T, the overall desired consistency is still quite dry - but you want some lumps. Sprinkle over the fruit. Bake at 350 - for about 20-30 until golden. Please note that this not a good dish when older then 2 days - it's not that it goes bad, but it goes either soggy or dry - so make when you have a crowd.