I don't have an MS20 on hand, so I can't be specific with details.
Somewhere in the circuit you should be able to locate the two amplifiers that boost and filter the PIR element signal.
It's very possible that you could cut the connection between op-amp #1 output and op-amp #2 input and insert an audio potentiometer as a shunt to ground.
To get an idea of what I mean, check the "basic motion sensor" schematic at:
Note the "audio pot" and 100k resistor shunt to ground at the output of op-amp "U1A". Also note that the input to "U1B" comes from the *wiper* of the potentiometer. This audio pot is the sensitivity adjustment. Use of a non-linear audio pot gives a more uniform change in sensitivity in relation to the mechanical rotation of the potentiometer.
You might also be able to fiddle with the feedback resistors of the op-amps in order to change the gain of one or both amp stages. Doing that will tend to change the "good" frequencies that the op-amps will boost, so maybe that's not such a good idea.......
I just thought up a really easy sensitivity adjustment that will work with any PIR sensor.
Find some thin plastic material that does not block the infra-red from reaching the sensor. Experiment with various types of plastic - some types block the IR, some do not.
Once you find some appropriate material, just add layers until it reduces the sensitivity to the level you want.
You can buy specialty plastics for this purpose from
< http://www.fresneltech.com >
It's UV stabilized and passes most of the spectrum required for PIR detectors. They do have a 30 US$ minimum order though.