I wonder, How much can these 8bits computers be pushed in terms of overclocking the cpu / replaced with a new cpu?

Depends on your definition.

If you don't care about socket compatibility you could simply load an 8-bit CPU in a fast FPGA. It will probably not be hard to run one at 100MHz or so.

Thanks. I meant in terms in desicated parts but FPGA might do it. I think we need some starter computers for kids and 8 bit seems quite a good choice. A lot of us started with them and they were simple enough so we could program them ourselves.

Grant Searle's breadboard CP/M machine: http://searle.x10host.com/cpm/index.html

Spencer Owen's RC2014 modular bus-based Z80 system: https://rc2014.co.uk/ Highly active RC2014 discussion group: https://groups.google.com/forum/#!forum/rc2014-z80

Phillip Stevens' beautiful YAZ180 Z180-based SBC: https://github.com/feilipu/yaz180

z88dk, a Z80 C cross compiler with libraries and all sorts of useful things, which supports many machines: https://www.z88dk.org/forum/

Jon Langseth's LiNC80: http://linc.no/products/linc80-sbc1/

Steve Cousins' many different machines, some RC2014-compatible, some z50bus (LiNC80) compatible, some Z180 stuff, SBCs, a powerful "small computer monitor", etc: https://smallcomputercentral.wordpress.com/projects/

Alan Cox's Fuzix, a unix for 8-bit machines (including many of the aforementioned Z80 machines): https://github.com/EtchedPixels/FUZIX

Most of the folks mentioned can be found on the retro-comp discussion group, which was formed from RC2014 group members to discuss other projects: https://groups.google.com/forum/#!forum/retro-comp