I've decided to see what it would take to make this oddball GBC game work correctly with the new SP. The tilt sensor is in the chip on the very top of the cart, just a bit to the left (you should be able to see it easily) It is a ADXL202 X-axis and Y-axis tilt meter (basically pitch and roll sensor). I have bad news and good news...

Bad news: since there's only 2 output (X axis and Y axis) it's not going to be easy to flip it.

Good news: you can desolder the whole chip, rotate it 180 degrees and then wire the chip pins to the original pad. Since it's rotated 180 degree, it'll work with the SP. But it will no longer work with with GBA or GBC.

Alternatively someone could get the ROM of the game, hack in to add a menu at startup to ask if the player is using SP or GBA/GBC and if it's SP, flip the control and then play as normal. But you'd need an EPROM burner and the resulting modified game cart would have ugly black chip sticking out and it'd be a bitch to make the cart fir back in the GBC.

Another thing, for using the game with GBA Player for the GC, just take the whole GC and put it in your hand with the cart slot pointing away from you. Tilt and roll the whole 'cube to control Kirby.