I got a non operational C16 on which I could determine the fault to the CPU.

It is possible to replace the 7501/8501 with a 6510 using an adapterboard. You will loose some functionality(or have to compromise) since the CPUs have different amount of Processor Ports. The Kernal ROM should also be replaced with one adjusted to the 6510 processor, but fastloaders(disk) for example will not function properly.

There is a FPGA replacement and a 6502 N-MOS replacement options available which allow for full functionality, but for basic repair/testing this adapterboard will work.

Sources: Hackjunk , CPU4C16 , FPGA