I've got a 93 (or is it 94, I forget) kick engine in my 90 tracker. You definitely want to keep the stock manifold though, the sensors will be different although I'm not sure the exact cutoff of what year etc. You can only swap the 8v engine though using your stock ECU as the 16 valve is not a TBI setup like the 8v. I believe any of the 8v 1.6's should fit without a problem though. If you keep your existing manifold as is, it basically is a bolt in replacement. I don't recall any significant things I had to change on mine although it has been about 3 1/2 years since I did the swap. Worst case you *could* need to drill and tap a few holes or something minor like that but I'm pretty sure that it was just a direct bolt in.
I'm usually somewhat leery of used engines since I don't know what the previous owner did with them. I'd say buy it and rebuild it if you have the time before you swap it out. There's nothing worse than swapping in an engine that wasn't in that great of shape. Sure,it's going to cost more to rebuild it first, but at least then you know things are done right. The only hurdle there is if you happen to have a block on the engine you're rebuilding that has the open top around the cylinder liners, you'll have a bit more trouble finding a machine shop that will bore it for you if that's the case.
My replacement block had the solid top but, the 90 factory service manual shows the open style block. If they give you any flak about it being an aluminum block remind them that the block uses iron sleeves so it's no different than an iron block.
All in all the 1.6 is a pretty easy engine to rebuild. Just make sure you get a factory manual for torque specs, I don't trust Haynes and Chilton much for internal engine torque specs etc.
If you go the rebuild route, look at maybe slapping in a better cam and cleaning up the ports on the head a little bit, that was my biggest gripe with the way the engine was put together. The block casting is pretty high quality but the head casting was crappy and had a ton of casting flash and other junk in it in both the intake and exhaust ports. Don't get too crazy if you don't have a flow bench, but cleaning things up a little isn't a bad idea.
One suggestion I would make is to look for a 93 or newer engine, the counterweights on the crankshaft are significantly revised compared to the earlier years cranks. The newer ones also use fully floating piston pins vs pressed in pins on the older ones. If you're putting a good deal of highway miles at relatively High RPM's it's not going to hurt to have the better counterweight setup for longer engine life as well as things like the crank seals wearing slower.
It might not actually make a big difference but it seems to me they must have revised it for some good reason since that would have cost a decent bit of money to do between design and increased material cost. It's pretty rare for a change like that to happen without a good reason. At the very least it probably runs a bit smoother.
