Hello Ian welcome to the forum. To answer some of your questions, my dad always says don't throw anything away.
The engine in a Mk1 Pi would be a CR prefix and a number below 6800 on a british registered, I'm not sure about the overseas numbering. Anyway the crankshafts in a Mk1 pi are unique and what they call a long back cross drilled, so these and the engines very rarely do they come up for sale, if they do the they are used in the TR5 models so they can be sort after and be pricey. So unless a piston is hanging out of the block it is worth holding on to the engine if it is the original one. Bearing in mind unless you change the clutch or main bearing oil seals you will never know if its a short or long back when insitu.
My was car was complete when I bought it and the list below would be my hard to obtain list.
and items which you want to store in cotton wool and don't break
Overdrive Column Stalk
Injection Bonnet Chrome Cowling
Air Filter Housing and the original grey pipes
Flat indicator lenses on the B post
nos wheel trims
Badges Bumper 2500 Badge and 2.5 Pi Boot Badge
Seats original ones, all of them hard to find.
I could do with a front grill, not looking forward to getting one of those.
Very early cars:
No air screw type throttle bodies.
I was given an incomplete early 2000, I put out a list of bits I needed and had no joy really, so up against it I sold the shell which was savable. I've recently found its fubar now.
unfortunately cant save them all.
If there is anything you need or reference pictures we're more than happy to help you out give us a shout.