it’s not a big deal blowing a motor. You can replace the engine and have the car perform the same. It’s not like frame damage or something.
however the kicker here, is that you seem to be implying that you didn’t ask your father to drive his car. Let alone take it to a trackday. That’s the shitty part. If this is the case, it’s not that you blew a motor; it’s that you blew your parents trust, and in a big (and expensive) way.
While I’d assume him to be pissed about the car, I think the disappointment of the situation would be the trust issue. Someone posted 21, I’m assuming that’s how old you are. As we grow up, people (esp our parents) expect us to act like it. when we do stupid shit we would have done at 16 when we’re 21, it’s a bit of a let down. Ferris Bueler was in high school.
There are a few things I would never do; take my dads vintage jazz bass without asking, and take one of his cars without asking. As I never did these things growing up, he let me borrow a '64 jbass indefinitely, and he let me take his 240z to a trackday at watkins glen.
The issue here isn’t the car, or the motor, it’s the trust. It’s broken, and I don’t know how close your family is but that is often a tough thing to truly get back. I mean obviously they will still love you, but fuck I wouldn’t look my son in the eye for months maybe longer if he took my car without asking, let alone took it and blew the motor.
Another factor is the wait time; say you tell him when he gets back. His first question will be “when did this happen?” I’d imagine it would just be another twist of the knife to hear it happened days prior, you two had spoken in the interim, and you didn’t say anything.
Even if it pisses him off and fucks up his vacation, its still the right thing to do to tell him as soon as possible. And as crazy as it sounds, manning up right away is also a step towards regaining trust.