You can get a 20MHz Z80 that will overclock up to 33MHz or so. A Z8S180 runs at 33MHz and includes timers, uarts, dma, and an MMU. I don't know how much you could overclock one of them, but it's already pretty fast for an 8-bit chip.
Zilog also makes the ez80, which is like the Z180 but runs at 50MHz and has a 3 stage pipeline, and it has a 24-bit mode.
On the 6502 side, there's the 65C816S which is the fully-static CMOS version of the 65816, which is a 6502-compatible CPU with a 16 bit mode and 24-bit addressing. The Apple IIgs had one, but its clock speed was limited to 2.8MHz so as not to compete with the Macintosh. Today you could build a machine with a 14MHz 65C816S and fast SRAM, and have a pretty beastly 6502-compatible machine.
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.
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/
People have run Hitachi 68C09Es at 5 MHz. May not sound like much, but the 6809 (which the 6309 is an improved version of) takes fewer clock cycles than Z-80 or 808x take to do similar work. OpenCores has a 6809 core, not cycle accurate, that can run at 40 MHz if I understand the description rightly (https://opencores.org/projects/6809_6309_compatible_core).
