--how to determine whether a low compression problem is rings or valves..
index all spark plug wires,so you can put them back the way they are supposed to go..usually, make sure they are in the wire looms correctly.
-2--pull ALL the plugs-this way you can count how many times the engine turns over..4/5 times is usually enough..
-3--check compression "dry"..without adding anything to the cylinders..while all the plugs are out,go ahead and check all the cylinders..usually a good cylinder should produce near 100 psi,depends on kind of motor,
--4--the cylinder with low compression-take and squirt a tablespoon or so of motor oil in the cylinder-easily done with a foot of vacuum-size-hose-small enough to fit in spark plug hole..dip it into quart can as far as you can-put your thumb over the end-put other end in cyl - this will temporarily seal the rings,and will raise the compression..
--5-- if this raises the compression-this means the rings are bad,and need to rebuild the motor...if it DOESN'T raise the compression,you have a valve that is leaking and might get by with having the heads reconditioned ..i agree with the other poster, you might be better off, and save coin by installing a long block, as everything inside has been reconditioned and you get a warranty! Either way, it's not gonna be cheap! Were you pulling a boat up a hill?,or did it leak out the coolant? Usually,you have to work a engine pretty hot to burn a valve or rings..I've even seen a hole burnt in pistons, mostly while out in California, where LA drivers would try to climb the hills going to Las Vegas in 100% heat-in high gear with the a/c on cause they were hot..well, by burning up their motor, and not downshifting, and turning off the ac while climbing a hill-they ended up a lot more uncomfortable-when they broke down - in 100° heat-in the desert!..we get spoiled with automatic transmissions and a/c,we can forget that some conditions require some skill.