While I doubt you have the money to spend on it, this thread reminds me a lot of the Matias halfkeyboard, as well as any number of more affordable external numpad devices.
I know [[url=http://shop.ebay.com/i.html?_trkparms=65%3A10%7C66%3A2%7C39%3A1&rt=nc&_nkw=keyboard microswitch&_sticky=1&_trksid=p3286.c0.m14&_sop=15&_sc=1]individual keyboard microswitches are sold too](‘http://[URL=‘http://[URL=‘http://[URL=‘http://shop.ebay.com/i.html?_trkparms=65%3A10%7C66%3A2%7C39%3A1&rt=nc&_nkw=keyboard+microswitch&_sticky=1&_trksid=p3286.c0.m14&_sop=15&_sc=1individual keyboard microswitches are sold too’’’’) although I’m not sure as to their compatibility for use with an arcade board, how you’d find a properly fitting keycap or if microswitches are even what you want. A majority of today’s keyboards, especially the flat buttoned kind with the short throw like Apple’s Wireless Keyboard use silicon membranes with metal contacts on the button to achieve their general mushy feel and short throw. This not only changes the feel but it also makes the circuit board on the keyboard a part of the switch.
The biggest problem, perhaps is that the keyboard itself has elements of the button shell built into the casing asis shown when the keys re popped off. You could theoretically cut them out but it’s definitely not just a simple salvage and reassemble…
If you’re just looking for something with the same basic tight layout as a normal keyboard, so your fingers can navigate the keys quickly, it might work out if a numpad could be adapted. However there’s not a whole lot of flexibility in this project to say the least.