How should I handle serious mechanical issues on a car I borrowed from my parents?

a while back i borrowed a used car from my parents , they had it for a while and weren't using it much and i needed a way to get to work in another nearby town. it was a 2015 and has had some repairs over the years but worked well until more recently.

it started to develop an oil leak ( at gasket ) and seems to have issues with breaks as well and some rust is showing on various areas , i got a quote a few months back to repair the oil leak and change the belt ( needed to be changed as oil got on it ) and was around $1500-$1800 depending on what they needed to do

i showed quote to parents who still legally own the car and they didn't take it seriously and claimed garage was trying to scam us and that car was not really in that bad of shape

a few months later car isn't any better , i don't even drive it on the highway anymore as i found faster speeds increased the oil leak and it do weird things once i got off highway so figured out some ( paved ) back roads i could drive instead but car isn't doing much better just surviving

i don't have any extra money to do the repairs and its not even legally my car i was just borrowing it , i do have caa ( roadside assistance ) but that only gets me a free tow to a nearby garage if and when it does break down

Updates
1 mo
been driving it around as is since i posted question , did finally get it into the dealership garage yesterday and they confirmed it has an engine oil leak and needs a part replaced and could be fixed for under $1000 according to there estimates , parents questioned if it would be worth fixing but have came to an agreement to get the car fixed , have to book another appointment but might have it done before this weekend
Updates
1 mo
finally went to garage today and got it fixed , no longer an oil leak , still have to go in again to replace belt ( which got covered in oil when it was leaking and needs to be replaced ) appears car runs mostly fine although still a couple minor issues and realise it won't last forever but seems like it might return to normal for a little while at least
How should I handle serious mechanical issues on a car I borrowed from my parents?
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