Could be a lot of things.
A slipped timing belt will throw the cam gear off a bit and will give you bad readings if you're doing a leakdown test without checking if the piston is in full TDC.
Cracked head will also give you bad readings.
Carbon buildup around the valves will also do the same, even though mechanically your valves are closed and there's no tension on the cam buckets at all.
Cracked ring lands and/or broken rings will definitely give you those readings, this happens when the engine's been driven hard and runs lean.
Head gaskets can still leak without any evidence of oil in water/water in oil, it just depends where the gasket blew out. Not always does it blow out next to an oil or water jacket passage.
Its obvious that you'll need to pull the head off to do a complete inspection, there's just no way around it at this point, as none of the scenarios for low compression, aside from a slipped timing belt, are repairable without pulling it off.
Take the head into a shop once you pull it off and remove the cam, have them do a full inspection on the head. You'll want them to double-check the compression in the bowls, this will ensure the valves are okay and sealing correctly. You'll also want them to magnaflux the head, this will check for cracks, which is what happened with a 3VZE I replaced a head on years ago. They'll also check the deck for warpage, and if all is well with the head, then the engine will be coming out for a complete rebuild, as it will be pistons/rings causing the issue.
Oops, forgot to add, if you can't do the work yourself, for this job you might wanna try a mobile mechanic, see what he'd charge to pull the head off for you. They're usually a bit cheaper than a full-fledged shop, and if its just a head gasket, they should be able to do the work correctly and for a reasonable charge.
I hate hearing people who want to buy a used engine and just throw it in their vehicle, expecting it to run another 100-200 thousand miles! If you get a used engine, you really need to rebuild it before it goes back into the vehicle, so why not just rebuild the original engine that you already have?
If there's one thing I've learned in life, its cutting corners will almost always come back to bite you in the ass! Spend a little more now and get it done right the first time, you can never go wrong with that!!!
