YZ450F engine reassembly question.

I bought an 08 YZ450 minus the head for the forks and a few other parts(the original thread is here http://www.vitalmx.com/forums/Moto-Related,20/07-YZ4fiddy-question,1331…). I decided to get it running instead and I found a good deal on an 09 head, it came today and looks to be in good condition. I can do 2 strokes or horizontal Hondas in my sleep, well at least I could 20 years ago. But I have never had a modern 4 Stroke long enough to do anything more than change the filter and oil. I've had one 400, and 2 450s that I sold with less than 10 hours, one with less than 3. Let's just say that some of my seat time the last 20 years has been very expensive. I'm sure my questions are moron level but I do know my way around the garage for the most part.

So.

What I have in front of me is a head that looks to be in very good condition, and a cylinder/bottom end that look good from the outside. But, something obviously happened with the head or it wouldn't be gone.

Ideally I'd like to put the head on and get it running to make sure everything is working, check compression and electronics before tearing it down. I'll assume tearing it down wouldn't be a bad move since I don't know any history on the bike.

It looks like you can access everything by pulling out the clutch. Do I dare put it back together with the old chain, I can't see any trauma but again, something happened with the head. I assume something beyond the chain is bringing oil up to the top, what should I check to make sure that's all working correctly.

I do want to tear it down if for nothing but to find out what extra parts I've got, the head came with what looks to be most of an 09 minus crank, rod, and right inner case. They changed the shift forks between 08 and 09 so I don't know if any of the parts will interchange but at least I should be able to figure out what they are. Just scanning the extras I got I probably have enough extras to get most of the $450 I paid for the head back, which gives me a hopefully running project for well under $1,000.

Yeah I could probably watch Youtube for a few hours and find my answers but I hate weeding through videos in hopes that I can find someone who know what they're talking about. Thanks.