I think first its better just to replace original quartz by 12Mhz (to have correct x2 timing; or if its 5.6Mhz in fact - by ~11.2); only then maybe (but not necessarly) You will need to replace cpu - and dont forget to make changes in pin scheme; You can replace ROMs with new/faster (just to have reserve) but keep old RAM as its required for correct work of device (or replace it with exact access time model)!