It might be the throttle body, or, just carboned up cylinders. I don't suggest a full blown Italian tune up, but you should see if you can get it to 4,000 rpm for a few minutes. 2nd gear is probably your best bet. Make sure the engine is fully warmed up first. After several 4K runs, then take it up to 5,000 for 30 seconds or so.
Check the crankcase vacuum with the engine warm, glove test. I assume this engine never got hot enough to burn off acids in the oil.
The vacuum lines, coolant lines, etc. need to be inspected and replaced if needed. Don't forget the brake fluid!
Start using synthetic engine oil.
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Keeping it running is better than buying new
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