I am the proud new owner of a 1980 Brat. I found that the compression on the number 4 cylinder reads at 120, the number 2 at 140, and the 1 and 3 at 135. This seemed like. Pretty worn motor, lacks power so I picked up a low mileage ea71 from a salvage yard. When I check the compression by hand cranking the motor (off the vehicle) I get 30 psi from all cylinders, except a 0 reading from the number 4. Does anyone have recommendations on what could be the cause, and how I should check and or fix before installing this motor? It has sat for 20 years on a shelf (hence the low mileage). Any thoughts or ideas are greatly appreciated, I love this forum!!!

Heads cracked between exhaust and intake valves? Fairly common when overheated

stuck valve? 

Had one with a blown head gasket do that also.

Turns out the intake valve is broken, and o or burnt. Either way, I figure this is why I get no compression in that cylinder. Any thoughts on whether this could happen on an engine that has been sitting for a long time, or is this a sign of a high mileage motor?

From sitting, no. I would question the salvage yard you got it from because it is pretty much SOP to compression test a used motor, or the yard just does not give a damn and lets the customer find out if it's good or not. No real loss on their end as they will either swap it out or refund, if within 30 days or so. I would not mess with it and return it as long as you got some form of warranty.