I had a similar case (original X68000 from 1987, with rust everywhere and a leaking battery), with (among other problems) a nonfunctional 69MHz oscillator.
I ordered a Cardinal Components' CPP series programmable oscillator, clocked at 69.552MHz, and after butchering my motherboard, my X68000 now starts and displays the "Please insert system disk" message
The oscillator replacement process wasn't simple, because on these programmable oscillators, the pin 1 is not the ground, but a positive Output Enable signal. So you'll have to find a clean way to let the pin 1 float (an internal pull-up will enable that signal). If someone knows how to properly do it, please share your knowledge.
I ended up using a cutter, a soldering iron at 350°C and several hex keys to file the hole on the motherboard and prevent the pin 1 from touching it, this is a complete mess but it works.
There MUST BE a cleaner way lol.
EDIT: here is a link to the oscillator data sheet: http://www.cardinalxtal.com/uploads/files/cpp.pdf
The "field oscillator programming instrument" is expensive, but Digikey (and probably others) can program them for you when you order the oscillator, without charge.