The design of the WS20E is very similar to the original speaker. The four screw holes in the metal version of WS20E are so bored that they come to lie exactly on the four existing screw holes in the Mission-707-front. Therefore, the screw / thread can be used further. However, the WS20E not fit because of its slightly more accurately in the basket 20-cm opening in the front. I had both holes with a milling head (in drill) bit wide, until the holes were large enough that the metal edges of WS20E speakers smooth on the respective front pads (about 30 minutes of work).
The data and characteristics of WS20E (impedance, etc.) seem good to the built-in crossover of the Mission 707 fit. The somewhat harsh suspension of WS20E is advantageous because the Mission 707 is a bass reflex speaker. My mission 707 boxes have since re-approximate the original sound. At first I had the impression that the centers come across slightly weaker. Since, however, I run the boxes with a new Pioneer A-30-K amplifier, which I highly recommend (see my Amazon review), they sound as before.
The installation of WS20E done five years ago (end of 2010). Since then, the boxes run without problems. The repair was therefore successful in the long term.