Hi Frieder,
There is a BSP for the TWR-K60F120M that would be applicable to your custom board. The difference between it and a K61 part is the tamper detect, but most everything else would be similar.
MQX BSP's are created only for the evaluation boards, so that is why there isn't a K61 specific BSP. But the K60F or K70 BSP (if you removed the DDR code) would be closest to what you're looking for.
Also take a look at these threads:
Creating MQX 4.0 BSP for K10
Re: MQX: Customizing a BSP for new hardware
The biggest stumbling block is getting the clock setup correct for your particular board in bsp_cm.c. That's where Processor Expert, or the bare-metal sample code, can come in the most handy
Regards,
Anthony