You overclock any system, you add coolers, and you kiss your warranty goodbye. It goes with the territory. Having said that, ceramic 68K processor variants are generally tolerant, it's just the rest of the design that can have problems.
Overclocks are hugely individual to the specific board, and somebody else with the same board might not get the same results.
GALS (Gate array logic chips) are not common now, but they can be got, and they don't need programming. They are specific chips for specific jobs. So you CAN replace them easily, the tricky part is finding the things working and for sale (or for swap). They are socketed for ease of replacement, but the only part "exclusive" to GVP is the EPROM. Everything else is stock. PALs are different, you have to burn the gates in, so they can be made "proprietary".
Read off some chip numbers, try Google or datasheets. That should tell you if it was standard component or not. True custom gate array chips were very rare - even CBM did not make their own, they didn't have facilities to make them. Chips like Gayle, for example. Too many connections for their chip making equipment.
