Two people are suggesting pressure washing the engine. I don't think that it is a good idea. I thing you are dealing with an engine that has had a few very elongated Oil change intervals. One person is suggesting that you go to a professional. I think that any professional is going to suggest you get another block, be it a short block or a used engine. If you watch The Car Care Nut, you'll see him drop factory short blocks into high mileage vehicles - of this brand - all the time. His results are excellent, and his thoughts are considered. I don't see a true professional taking a chance with that vehicle... there is no guarantee of success.
I think that you are unlikely to get good results with chemicals alone. If I had this vehicle, I would start with a toothbrush, popsicle sticks, q-tips and microfibers and physically clean as much as possible under the valve covers and the oil pan. I am assuming that you are starting with some dissassembly completed. If that is not the case, I would absolutely run 25% MMO for a hundred miles, and use an engine flush like Liqui-Moly. When cleaning I would use gasoline or kerosene as a solvent, and probably use MMO as a chaser. Once physical cleaning was done, I would run kerosene through the engine, several gallons, top to bottom, Put it all back together and go onto a series of short oil changes -- thinner, cheaper oil for maybe 500 miles.
As others have noted, there is likely a blockage somewhere. Not really sure how you chase that down without pulling the engine apart.
There is a good possibility that this will not work, and a non-zero possibility of destroying the engine as you try to fix it. Maybe you luck out and drive off into the sunset. This is rambling, but I think we can all agree that there is no good news here, excepyt that there is nothing to lose.
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