Anything will fit as they vare standard except for a spacebar if you get a new spacebar and want it to fit pm me and i can do the mod.
Not really, just found out that their Ctrl, Alt are also nonstandard size(following the size of "Tab" and "|\"). Those few keycaps which following Cherry Profile(like noppoo choc mini, KBC POM and of course Cherry themself) are slightly "shorter"(in height) than Razer's original keycaps too. Though if you using the keycaps for the whole boards, it will be just fine.
KBC POM
BTW, how do you modifier a normal spacebar to fit Razer BWU? AFAIK, the one on Razer is shorter in length?