Well. Since finding a new motor turned out to be a challenge, I took the old motor apart the other day to see if it could be salvaged. No such luck. One of the windings in the stator has overheated, probably shorted, which explains the smoke coming from the switch.
But today I was in luck. I asked at a garrage, where they sent me to a salvage place (AHKA on Ole Romersvej 16, in 2630 Taastrup, for those of you in Denmark). I brought along the old heater blower and housing. We managed to find one from a Nissan Primera 1995+. It had exactly the same diameter on the blower wheel. It is a one speed, normally using external resistors. However I have bought the electronic motor controller that Oetker pointed me to, so I won't be using the switch, but mount the knob on a 22Kohm potentimeter..
I bought the blower, realizing that it would be a tight fit, as the motor protrudes a couple of centimeters further than the original.
After removing the blower wheel from the new motor (it was oriented to spin the wrong way) I gound a flat surface on the shaft so the original wheel could be mounted (don't push it all the way onto the shaft or it will drag on the motor mount). After testing the motor (4329 rpm at no load), cleaning and lubricating it, I inserted it in the housing and did a test fit in the blower compartment.
It did not fit.
The corner of the inner fender was in the way, but after removing a bit it still would not go into the compartment. The motor was 1+ cm too tall. So, I cut an opening in the back of the motor case (plastic) and found that it did not support the motor in any way. Encouraged, I cut off about 1,5 cm of the housing and sealed the end with duct tape held in place by a strip. Since the shaft stuck out a little bit I placed a plastic cap over it before applying the duct tape. The cap was not tall enough, so I ground off 2 mm of the shaft that stuck out. Success. The fan now slid into the compartment without problem.
One thing to realize is that the new motor assembly does not have holes to allow the air into the Murena housing. I had to cut them with a hole saw, but that was easy.
After mounting the fan and connecting it using the original connector (just two wires, but remember to swap +/- because the motor needs to spin the opposite direction) I jumpered it to the battery. It draws 10,7 amps and creates half a hurricane in the car.
Tomorrow, if I get a potentiometer that fits the dash, I will mount the motor controller and wire it up to the fan.
Pictures below.