Just got a X68000, so I thought I should join here and peak around. So I bought a Expert on YAJ a few weeks ago and it had a overclock on it, so I opened her up and wow is it ghetto as hell. Oh the board wasn't sticking to anything..it was just flinging around lol.
http://i1294.photobucket.com/albums/b606/hyperneogeo/101_0967_zps88416871.jpg (http://i1294.photobucket.com/albums/b606/hyperneogeo/101_0967_zps88416871.jpg)
Not surprising really. people find a variety of overclock mods fitted to there X68ks from time-to-time. some of them well done.. others.. not quite so.
The Expert model is the highest overclockable one of them all for some reason, with an optimal overclock of 17MHz. Though I've read on some obscure Japanese computer sites, some people have managed to get them as high as 20Mhz.. but not without stability issues effecting them though.