The original Einbeck beer has 1/3 wheat malt, 2/3 barley malt (lightly kilned for the time) and was very bitter with a slight, but pleasant acidity. It was also apparently top-fermented (but brewed in winter), but cold-conditioned (lagered... like an Altbier).
To get the right taste, you'll need to have some acidity. You have two options: 1) The easy way. Buy some Lactic Acid from your local homebrew store and add it at bottling. You'll have to add it to taste, but I'd think about 1/8 oz or 1/4 oz would work. 2) Sour Mash: Make a solution of sugar water that's between 1.030 and 1.050. Add two tablespoons of any malted grain. Let it sit at warm temps for 24- 48 hours or so. Strain out the grain and add it to the boil. This will grow some lacto bacteria, but kill them in the boil
Einbecker was stored cold (lagered) before serving. If you have the means, cold condition (35 to 40 degrees) it after it has fermented out for 2 weeks to 1 month