The engines are totally stout. They have huge valves for a 1600 - 1800 and a great short block. I've built 11 or 12 twincam engines. There are subtle differences to the engine in yours (a 1608cc if original), vs the '73 1600 (1592cc) and the later 1800-2000 engines. Parts swap easily between the 1592 - 1800 engines, particularly the 1592 head which have no EGR passage. Depending on who you talk to, the 1600 to 1592 or 1800 swap possibilities are more limited (revised water jacket openings, etc.). However, the Italians assemble cars out of whatever happens to be laying around so it's possible your car may have some unorthodox stuff in it from the factory.
If you decide to throw in an 1800, be aware they changed the bore spacing of the block but did not change the combustion chamber spacing in the head. So two cylinders and their valve gear are slightly off center.
Make sure you change timing belts every 25,000 miles. It's an interference motor no matter what the year. This Lampredi engine design is the first commercially available vehicle with a timing belt so they're harder on belts than anything coming after them. Just change the belt and the tensioner too.
The trannies cannot be killed except if the trans pan gasket goes south. Then they lose all the lube quickly. However, I ran one dry, fixed the gasket, filled it up with lube and it worked fine.
The starter has three bolts. Reaching the third bolt requires two elbows in one arm so it's a test of your manhood.
the Naugahyde used in the interiors is fairly robust but they sewed it all with cotton thread. That thread rotted 20 years ago. There are kits available, however. Parts support is very good.
This is a car that often commenced rusting in the showroom. FIAT was forced to rebate $1200 - $1500 per car to buyers by the Office of Consumer Protection. There is no Italian steel industry. The steel used in this car was all recycled and badly at that. So they can be rust time bombs. Having said that, the worst have already dissolved so the remaining cars have a little better metal in them. Its been years since my involvement with them but someone else can comment on sheet metal availability. There was no aftermarket sheet metal when I worked on them.
The big electrical problem with these cars is bad grounds. All of them should be checked and improved if necessary, especially if the car is rusty. If you lose the big ground strap between the starter and the body, it will ground through the clutch cable which makes for a very interesting situation while driving.
Look for excessive negative camber or limited turning ability to one side. Those are dead giveaways for fatigued / rusted metal messing with the suspension / steering geometry. I bought a parts car where metal fatigue was so bad, It would not turn left more than a few degrees. I picked it up in Weymouth, MA and drove it back to Newton. Since it could not turn left, I had to drive it in concentric circles all the way home.
Having said all that, I had an absolute ball in those cars. I courted many women in them and created a little side business fixing their problems and rebuilding their engines. So I benefited greatly from my FIAT association in many ways.