1999 YZ400f needs new engine want a yz450f motor

I have a friend with a "99 YZ400f, it needs a new bottom end. There was a '98 YZ400f that sold locally near me that had a newer 450 motor (can't remember if it was 03-05 or 06-09) swapped in. I was under the impression that this was not a bolt in swap? the swing arm on the later bikes is different? can I just get a swing arm from one of those bikes and put it on my friends 400f? basically what all is required to put a later 450f motor in a 400f bike?

The '03-'05 motor will nearly bolt straight in, but you will need to address the swing arm pivot issue. The steel inserts at the rear hard point are removable, and they might interchange between the two engines. At worst, you can have a set made for you with the smaller bore size for the early pivot. Changing swing arms won't work because the holes in the frame won't match the newer one.

'06-'09 engines can be made to work, but they introduce carb clearance problems and take a bit more work to fit in. They also use a completely self contained oil system, so you would not use the tank in the frame.

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