There's nothing really available for either the CA20E or CA18ET (the only engines available in '85). You can get a header for the CA20E, or a cam.. the CA18ET is a turbo, of course, but it can't handle much boost. It only ran something like 5psi stock. The CA20E could be a nice engine if people bothered to make some parts for it, but the SOHC CA motors are just orphans as far as that goes.
CA18DETs and even SR20DETs can be swapped in, but I wouldn't suggest it unless you have the FSM and enjoy wiring because it's a bit different from the S13. One benefit over the S13, though, the US-spec S12 is designed to mount the ECU on the left side of the car (same place as JDM S13s, opposite side of US S13s) so the engine harness doesn't have to be cut and lengthened to reach.
Suspension parts are tough to get... they're NOT directly swappable with the Z31 despite what anyone says, as far as I can tell the lengths and such are different even though the suspension layout is the same. If you can manage to dig up a set of good front cartridges and rear shocks, Ground Control's sleeve coilovers work great.
Brake upgrades are where Z31 300ZX parts really come in handy.
The turbo models have a lot of cool '80s-ness that they share with the Z31... they talk, they have an alarm clock in the dashboard, and a FULLY digital dash including a bar graph boost gauge! <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/thumbs-up.gif" border="0" valign="absmiddle" alt=':thumbsup:'> Also the factory stereo's optional tape deck fit into a second half DIN slot below the stereo slot.
If you need any S12 parts, seek out the cheapest, most ghetto u-pull-it junkyard you can find. They get S12s by the truckload, often turbos or V6 models.