to rule out the dynastart you have to bypass other elements that have a tendency to fail.
when you turn the key, power should go to a starter solenoid, if the points on this are mis-aligned, burned whatever, it will create more resistance and not allow enough juice to get to the dynastarter.
you should try jump starting the engine by finding the starter post (don't know where it is on a Heinkel engine) but a thick cable that goes into the casing via a post connector of some sort.
apply positive cable to this post and another to a good ground on the case itself.
if the engine is not in gear, it should crank. If it cranks the same way it did when you tried it with the key, then you know the problem is inside the engine someplace related to the dynastarter unit.
If the engine cranks fine when you "jump" it like this, that means the "problem" is most likely a bad ground some place or the voltage regulator/starter solenoid assembly needs to be gone through.