First of all, please note:
View attachment 82682 This is the normal ignition coil wiring scheme. (The transistor switch could also be in the ground leg of the primary of the ignition coil)
The schematic you provided (presumably from the manufacturer) is incorrectly wired.
That said, it strikes me that it would be easiest to interrupt the CKP sensor's signal, with a 555 circuit as you suggest, using a transistor switch ( no "bounce").
There are, however, some timing issues that you'll have to consider:
1. if you interrupt (as suggested above and by an obviously random button push trigger), the spark trigger or for that matter, the spark itself, there may
still be a combustion sequence in the cylinder if the interruption occurs
after the spark sequence has started.
2. this will affect the "unloading" of the gearbox, thus interferring with the shift process.
3. as a result, you'll have to arrive at a method that is timed to interrupt the spark just
before the spark is initiated (by whatever method).
Of course, even just a
lessening of the energy of a combustion stroke may be sufficient to achieve the unloading of torque you seek.
(BTW, "The Worlds' Fastest Indian" is a fav movie of mine...)