Not an expert, but it could be a variety of things. Since it starts right up, the injection timing is probably not way off. If it has low oil pressure, the main bearings/conrod bearing are probable worn out/damaged. Could be a loose of damaged valve train part. Might just be something unrelated to the engine just rattling. Hard to say.
Whether it is worth it or not depends on what you are paying for the grader, how much more work it needs and what you plan to do with it.
If the engine is on the way out, it will probably cost you $4Kish to replace it, if you do it yourself.
The JD 329 engine and the 570A grader are both supposed to be very good products.
