> I'm trying to write an 68k emulator and I'm wondering about a few things.
>
> * Is there any reference available where I can find how many cycles the
> instructions take? I couldn't find it in the motorola programmer's reference.
> Can I cheat here a bit?

There is documentation, a document (available as .pdf file) named M68000 (fill in the rest of the series too) 8/16/32bit Microprocessor User's Manual. It has all the timing info, have fun with the multiply instructions (as soon as you see the description for those, you will start to cheat a bit). ;)

> * Since I want to use it primarily for arcade emulation, are there some things
> in the 68k which are almost never used (so I can make life a bit easier at first
> :-) ?

Yes, you can avoid dealing esoteric details like the way multiple memory spaces are supported without any adverse consequences for arcade emulation (or 99.99% of any emulation). You can also neglect the prefetch on the 68K (though you'll need it for e.g. Amiga or Atari ST emulation). You can avoid other (fairly) esoteric stuff like trace mode or alignment exceptions as well.

> * What is a good arcade game to start with, so a simple to emulate yet fun
> machine? So that I won't have to torture myself trying to get Galaxy Force to
> show anything, you know... :-)

I wouldn't know a good example, though there definately are some. Maybe Biomechanical Toy?