Having the 68000 run any faster would only speed up a few instructions taking several cycles, the rest of the time the CPU would be waiting for the RAM. Without any form of cache or fast(er) RAM it'll be pretty pointless.
The only way is to speed up RAM as well - and since it's chip RAM, you'd need to overclock the whole chipset. Provided your monitor can cope with that (multisync, no 1084 derivative) you can replace the main crystal with a faster one. I've tried a 32 MHz crystal once (A500) and my monitor was whining terribly, so I stopped after seeing it boot.
(I had a sync switch installed to be able to use composite input without unplugging RGB and used that to run the monitor unsynced for most of the time - wouldn't recommend letting it run for more than a few seconds on 17+ kHz, if at all.)
