So, a few months ago my sister's 1997 Geo Tracker LSi 1.6l SOHC 16v severely overheated. Ended up being the typical blown head gasket/warped head, caused by a cracked radiator.

Anyways, 4 months later, and everything is repaired, replaced, and back in the car. Just finished it up today, and when I went to start it up for the first time, it immediately started, ran, and idled at approx 800 rpm. That's not having been run in almost 5 months. Pretty sure it should be around 11-1500, no? Anyways, everything looked ok and I couldn't see anything wrong, so we took it for a test drive. Drove great, had full power, nice and smooth, warmed up ok... then started to idle at around 500 rpm once it was fully warmed up. Seemed like it was barely keeping itself running. Turned it off to check the fluids and coolant level and everything, and when I tried to start it again, it wouldn't even idle anymore. Will fire up and die immediately, maybe stumbling for just a second. It will run and idle if I keep my foot on the gas, but won't idle on it's own. Haven't had the chance to let it cool completely down and try again, but I feel I may have just overlooked or missed something upon re-assembly, even though I can't see anything.

I am getting two trouble codes:
P1035 (which I believe is the O2 heater circuit)
and P0751

Again, I don't see anything wrong our out of place, no leaks, no misfire, everything seems fine (aside from the codes). Just won't idle. Any ideas? It's my sisters car and it's a few cities away now, but what should I check for when I go back tomorrow? Any help is much appreciated.

Disconnected or mis-routed vaccume lines, Timing, fuel pressure, anything you disconnect to R+R the head. That's just where I'd start. 800 rpm is the normal idle speed for one of these once warm, but warming up anywhere from 1000 to 1200 rpm. Look at your airbox and intake connections too. Hope this helps.

Yeah, I thought about all that stuff. Not to concerned about fuel pressure, I didn't disconnect the fuel lines. Just unbolted the intake, pulled it back a bit, and just set it aside. What I am concerned about is misrouted vacuum lines. Pretty sure I got them right, but I could have mixed a few up. Are there any diagrams or pictures of this engine to compare to? Guess I should have taken some "before" pictures when I first started, haha.

There should be a diagram of the vaccume lines along with all the tuneup data under your hood on the bottom side. If the tag is gone or not legible you can look them up here on the forum, using Ack's facts or the online manual. I have a 95 OBD1 Tracker with the 16V engine but  I think the OBD2 ( yours ) may be different with several more lines. Maybe not. I'll take a pic of my diagram later when I get home from work.