Structural body rust damage required 50% new metal. New panels much more resistant to rust though so worth it.
I have blown up 5 triumph engines since I have driven the car! but they are very quick and easy to replace. The engine oil in triumph spitfires overheats especially when in slow moving traffic the result low oil preasure and extreme wear on the big end bearings. Fitting an oil cooler is essential if you want an engine to last for more than 10,000 miles! I wish I had fitted an oil cooler with the first engine for it was the fastest one.
I have replaced the gear box for a better one, but later type gearboxes give little trouble.
I have replaced the entire braking system except the pedal. I used high specification parts. The rear cylinders will always sieze if you leave you car laid up outside over winter. Replacements are inexpensive however.
The electrical system took years to finally fix. Always carry spare reel of wire in the boot.
When replacing the starter motor the altinator and distributor I fitted modern equivslents. This by and large has made the car a reliable runner.
After having the whole right side of my car torn off by an encouter with an out of control combine harvester in a narrow lane my car is a little like robo cop or darth vader. It's made up of mostly replacement parts!