Retail value is probably somewhere between $8000-$12,000.
Trade-in value on a running, working average Aviator probably going to be $4000-$7000
If there is $4000 worth of work to fix the engine, then trade-in value might be $0-$3000.
Here is a link that details what is involved in doing an engine swap on an aviator.
Cooper Lake Automotive-Smyrna-Marietta-Kennesaw-GA-30082-770-431-1936 Lincoln Aviator Engine Replacement 770-431-1936
I can't comment on how many hours the job is, but lets say it's 10 hours (pbbly more like 13-14hrs) at $125 an hour, that's at least $1250 in labour just to pull and replace a motor, not including time to FIX the motor or the cost of replacement parts or replacement engine. Total cost will probably easily be $4000 to fix at a reputable shop.
The big question is WHICH cylinder was low on compression, and WHERE is the leak coming from? Plug, intake valve, exhaust valve, or piston ring?
If the leak is from a frontal cylinder and from a spark plug thread, you may be able to fix with the engine in place relatively easily / cheaply. If the leaky cylinder is near the back, or, if the leak is coming from a valve seat or cylinder ring - the engine most likely has to come out.