Few comments -
Thread response #21 - The car seems to be on its side .... I thought us southern hemisphere types were the only ones with issues like that, only our stuff is completely upsidedown
#22 - The engine looks nice and seems to be fitted with an alternator rather than a generator (dynamo). All good though you'll have to do a little bit of rewiring.
#23 - The starter is a cable operated one. The starter on my donor 650 was solenoid actuated. Fiat 126s may have been fitted with cable operated starters, I'm not sure so I'll defer to others on the list who know for sure. I guess if it fits the bellhousing, it's the correct one.
#24 - Carburettor gaskets and clutch throwout bearing. If your engine has been rebuilt, these parts should now be OK and not need changing. As the man says - they are 'ekstra'.
#25 - 126 bellhousing. Yep it's the correct part and yes your can now install your new motor. You will have to change the 126 rear mount studs for the longer ones from your old 500 engine. I think the only other minor issue you may have is with the warm air to the cabin. The outlet on the 126 engines faces a different direction to the one on the 500 (not a big problem) but the starter motor may now foul the metal centre piece between the two pieces of air hose. A bit of minor panelbeating should sort the problem.
Keep the pictures coming,
Chris