It really sounds like it's not getting proper fuel or the fuel is injected at the wrong time.

So, if it has not been maintained, odds are that the IP timing has never been set and it's probably five degrees late. This will affect performance.

But, the engine seems to be even further down on power than what IP timing would suggest. Any chance that the tank screen is partially clogged? Might be a benefit to drop it and make sure that it's clean. You can do a quick test by running the engine from a can of fuel under the hood. Or, you can blow compressed air back through the supply line to temporarily provide some additional flow through the screen.

What's the blowby? Is the engine quite tired or does it appear to be in good shape?