As jt20 said, the 601 engine rarely has valve or seat issues.

The 601 has an iron block with an aluminum head, so.....head gaskets will fail. Typical is failure between cylinder 1 and an oil passage or the front timing chain passage, which dumps oil into cylinder one and out the tailpipe. Non stop blue smoke. Been there, done that

If the oil has not been changed regularly over it's life, bad for the rings. I would do a compression test.