Can I replace the head on my 200SX while still in the car?

It sounds like a leaking head gasket. You will either want to have it fixed or replace the vehicle as it is not a cheap fix.
Thanks,
Lee

No check engine light?

this could be a head gasket poped or even a plug loos,,
best thig to do is a compreshion test first if there is no or little compreshion on a cylinder check head gasket

Replace head gasket.

sounds like a head gasket or a cracked head,

There should'nt be any fuel in your oil, it sounds like you might have a blown head gasket. If you think it might be overfuelling then remove the injector clips (don't mess up the order) and pour some fuel down the manifold and try start it. If it does'nt give a good effort at starting then I'd say you need to run a compression test. Sounds like you have a cracked block, warped head or bad head gasket.

Does it leak coolant?

have the linkage reset. its not very hard to do.

the starter on my 97 200sx is in the bottom-back-middle (a real pain in the a$$ location). spent about 6hrs doing it myself. btw I would suggest not buying a "cheap" starter like I did and had to replace it 6 months later.

http://www.2carpros.com/car_repair_information/year/1997/make/nissan/model/200sx/1997_nissan_200sx_drive_belt_routing_diagram.htm