Because of the age it might well be written off,
Our T reg cougar that was there was waterlogged as above but drove it home about 8 miles & she never missed a beat .
I think once it dry's out & a good valet to get the river water smell out that would be fine, but the insurance want to write it off...
Think we will see what the insurance offer us as she wanted to change the car this year anyway
The engine bay should be fine, if the engine is soaked obvisouly it would need to be drained inc. oil change, new plugs.
I think the whole interior might need to be taken apart to correct shortcircuits and cabling. Depending on the dirtyness of the water all inaccesible areas (like back of dashboard, etc) would need to be either taken apart and washed, or rinsed with clean water.
The seats would need to be ripped out and leather cleaned. If the water is especially smelly the seats would need to be taken apart and new foam lining would need to be used. Perhaps cheaper just to get new seats or to just do nothing about them apart from leather wash and recondition.
Carpeting would need to be ripped out as it's likely to not be straight anymore after drying, let alone smelly and mouldy. The groundwork would need to be cleaned and checked and proofed for rust. Then it would have to be recarpeted. Same with the boot probably.
The fabric roof would need to be taken off and cleaned properly to get rid of the smell.
I'd probably steam clean and grease all moving parts again aswell.
I don't know, if they love their spider then spending the money should yield them a pretty much brand new car again, in terms of interior, etc. But the hassle might not be worth it.
In anycase, I'd hose it down with fresh water from a fire hose let it dry and see what the damage is first.