Cylinder Break-In:
How many hours does the engine have on it and what is the oil consumption? Has the engine had oil fouled spark plugs? Higher than normal oil temperature? Since the engine has been recently overhauled, the cylinders may have not broke in correctly, which would cause the above as well as low power (because of the excessive blow-by past the rings).
When an engine is breaking in after an overhaul, the rings are setting up a wear pattern with the cylinder wall. In fact, that's why Mineral oil is used during the break in period for an engine, which increases wear to assist the ring and cylinder wall break in. After break in is achieved, normal Ashless Dispersent oil is used. Contrary to popular belief, cylinder walls are not perfectly smooth, they have very small ridges, which hold oil and this oil help lubricate the rings. Durning break in the top of these ridges are worn off and the bottom is left to hold engine oil.
If the engine does not break in correctly, too much oil stays in these small ridges, and over time the oil is baked in the small ridges and ring is now riding on a perfectly smooth surface, which is NOT a good thing. When this happen the oil control ring removes all oil (none is left behind because there are no ridges to hold the oil). This is called glazing. This can cause low compression, low power and high oil consumption. The only way to rectify this is pull all of the cylinders, hone them, and reattempt the break in.
Ignition:
Do the mags check good? What's the drop? If there is a large smooth drop, the mag is probably internally mis-timed. If you have a rough drop then a cylinder is dead (ignition wise), which would be a dead plug or bad harness.
If the mag drop seems good, this still doesnt rule out ignition until the tech checks the mag to engine timing. A grossly mistimed mag can reduce engine power (and if the timing is retarded would not cause a rough running engine, unless it's waaaay off).
Fuel and Induction:
Does the engine run rough at low power settings? If so, there could be an induction leak, or the primer system could leaking.
Camshaft/Wrong Pushrods/Lifters:
A extemely worn camshaft or lifters would cause and engine to not make power, however, this is extremely rare with a new regularly used engine (usually corrosion begins the high wear).
However , if the wrong length pushrods were used during the build up, the valve will not open as far as it should, causing the engine to make less power. Have the tech conduct a tappet clearance.
Burned valves are unlikely unless the engine has been REALLY abused, but its easy to check, while doing the compression check, listen for air hissing out of the exhaust. If there is a lot, a burnt valve is the culprit.
Obviously all of the above items should've been checked/replaced/addressed in the overhaul (OH) but you'd be surprised what you DON'T have to do according to the FARs to call and enigne an overhaul.
Hope this helps!!!