Should you buy Nissan Z24 engine for 650.00 or fix head gasket and possible cracked head or block on 1988 Nissan 2wd pickup?

I would bet it is just the head warped. Pull the head, have it checked for cracks and planed and put it back. I fixed my own by sanding the head against a piece of slate I knew was flat and sandpaper used for sanding floors. You can see the high spots and you sand till they disappear. Drove it for years and would still have it, but it rusted away. The head is the weak spot and is almost always what goes bad by warping.