PI-SIM Board mod - "The PI-SIM board is designed for Stern's ‘The Simpsons Pinball Party’. It has been built to allow the mini flipper bats of the upper playfield to be triggered when required only: when one or several balls have reached the upper playfield".
http://www.flippp.fr/pisim.php

Make sure you check your CPU first to ensure it's compatible. Per his site "The PI-SIM board only works with this type of CPU board: "520-5136-16 Rev. E".

UPDATE: I sent an email to Pascal asking if his mod (PI SIM board http://www.flippp.fr/pisim.php) would work on other versions of the CPU and here is his response:

"there are 3 CPU revisions for your game (TSPP)

http://www.pinwiki.com/wiki/index.php?title=Sega/Stern_White_Star_Repair

refer to chapter "3.1.1 CPU / Sound Board" :
- top right : 520-5136-16 Rev. E (Monopoly) => the one we currently have in our TSPP
- top left : 520-5136-16 Rev. D => same one with
additional logo "SEGA" instead of "STERN"
- bottm center : 520-5300-00 Rev. A (Lord of the Rings) => the newest of all 3 which causes trouble with CN5 farther from CN6

my PI-SIM board plugs into CPU 520-5136-16 Rev. E, and NOT on CPU
520-5300-00 Rev. A

we studied the cost to make a specific model for that CPU.
but a limited PCB batch would cost a fortune, and we don't have this CPU for trials.

a possible mod would be simple though: build "connectors extenders" for
CN5 CN6 and CN7, and bend them apart to fit that CPU's specific connectors spacing.

Pascal Janin"