The nearest model is the SL1800MKII which has the same motor, arm etc, and no extra bits to do any automatic functions. However whether the circuit-boards etc are the right shape, drilled in the right places etc, to fit into the case of the 1200MKII I don't know. I'm not sure it would be worth the effort to be honest, over using an 1800MKII as it is. The same goes for transferring the 'innards' of other turntables - I'm not sure you'd get a great sonic benefit over using the original casing.
If you're prepared to do some drilling, filing, re-wiring etc, and it's mainly an aesthetic result you're after (ie it must look like a 1200MKII), then the visible parts from a 1600,1700or1800 (all MKII) would have identical appearance to those from the 1200MKII, as well as being mechanically and electrically very similar.
Good luck with the project