I agree with the others that it could not have happened that way. However, the garage mechanic may have told you that hoping you would believe it.
First there are companies that build rebuilt engines for various makes and models, they take the old one out and pop the new one in. I would not have the existing engine fixed. In a garage you'll pay retail for all the parts, the labor costs will be high. Having said all that $8,000 is ridiculous even if you have a mechanic do the job. Shop around, there are honest mechanics out there.
It's not unheard of for the bolt to jar loose a little bit and the oil to drip out slowly.
Secondly it sounds like the warranty has expired, so you might want to put in an insurance claim. Even with a deductable of $500.00 or $1,000.00, it is what insurance is for. Blown engines happen for a variety of reasons, it won't be the first claim of this kind the insurance company has heard of. If they ask you to shop around, call or visit at least three repair places and get the quotes faxed or write them down and record the phone number of the place and the person you spoke to along with the quote.
The insurance company will pick one that charges a fair price, after all they have to pick up the rest of the tab. If they increase your premiums next time (most won't for something like this because it wasn't your fault) just shop around when it comes time to renew. I had my first accident a few years ago (my fault) and my premiums were not raised. My Dad had an accident the same year, his fault as well. His insurance company jacked up his premiums by $800.00. He called his agent and said the company had been taking his money for 40 years and this was his first claim. He called around, he was up front about the accident and found a company that gave him the same coverage for the same amount.
Anyway, some problems are made to sound awful when you talk to some mechanics. But it pays to get a second or third opinion, most mechanics are honest.
A "new" rebuilt or rebuilt used engine is really your best option. There are consumer organizations that rate the products of the rebuilders, it is wise to check out the company before you buy.
I looked on the internet and found a company that offered for sale a rebuilt engines for a 2000 models. The average price was $1560.00.
If your deductable is $1000.00 all you will pay is the $1000.00, if it is $500 or less, then you are laughing. But the mechanic doesn't sound right. Car manufactures stopped using timing belts about 20 years ago, it's all electronic.
I don't know if you remember but a couple of years ago a certain foreign pick up truck company had problems with the bolts on the axle assembly cover. All four could be shaken out over time and then in a moment the cover would fall off, some road junk would get sucked in and did all kinds of neat things to the axle assembley. Since pick-up trucks need lots of power, the strain caused other parts to fail and it was all due to substandard bolts.
Anyway, if you do decide to use insurance, they will be familiar with the problem and will probably be delighted when you suggest junking the engine instead of repairing it and replacing it with a rebuilt one.