I found a solution at a boat shop here in Germany:
I don't know how this thing is called in english, but it with it, i can mount a 700size Motor with a 30mm pattern to the 25mm pattern motor plate of the E-Maxx. The KB45 has got 9mm more in diameter than my 8XL and between the tranny and the 8XL is a space of 5mm, so the pinion and spur will mesh properly.
I'll order a KB45-8L right now and if it arrives and i have grind down the shaft to 5mm, i'll tell you if and how it works with a MM on 4s Lipo!