i have a NEC AMD K6 2. If i need to replace the motherboard, what would fit? What cpu should I buy and the memory as well? Or what else i have to replace to make it work faster and efficiently? Thanks

That isn't enough model information. NEC made some odd form-factor cases and proprietary motherboards that nothing else will fit.

"Or what else i have to replace to make it work faster and efficiently?"

The best way to repair something that old is not to spend any money.
Computers better than that are discarded every day, so I suggest looking for castoffs and trying to make one good computer out of the pile. It's good training. If you can find a business that tosses older machines in the trash (like a computer shop that handles business accounts) you might explain you are looking for something to learn on.

NEC made some K6-2 systems in cases about the size of those older emachines that had cyrix or K6 cpu's. They had the standard rear port configuration except there was only 1 USB port. The other was cabled to a port on the front panel. The PSU's were standard size which made them easier to replace than the smaller emachine types.

If that's what you have, most of the smaller form facter motherboards would fit, although you'd probably have to replace the rear port faceplate. And you'd probably leave the front USB port disconnected.