If it's a main bearing, crankshaft, it's life is very limited.
It could ceaze up at any time.
It is very unusual to run bearings in this day and age, a noisy gudgeon can sound very much like a bearing.
I would get a second opinion, if they decide it's still a main bearing, then flick a second hand motor in.
I buy and fit good quality 2nd hand motors for around $450 for my cars, and can remove and install in a long day.
Wreckers often offer an exchange service, buy a motor from them and they will fit it for you, usually in a day.
Try this, when you are driving at 60 kmh can you still hear it knocking loudly? if so it's a bearing, if you can't, it's a gudgeon, and could last for ever.
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