The previous owner of Rasputin thought that a good way to block the rust holes/perforation was to weld blobs of weld into the rusted metal... and then paint over it with underseal. To be fair, it lasted a while... just until the oil lifted the underseal and made a break for freedom....
Anyhow... new sump job:
Make sure you get the right sump for your motor. There are 2 different hole patterns/sizes for the 1.2. The advert will say something like "10mm holes"... or maybe just quote a number of holes.. so you'll have to get under the car to have a look. I unscrewed one of my bolts to measure it up.. it won't leak/fail as long as you put it back again afterwards.
I don't recall removing the bumper to change the sump.
You will have to remove the exhaust manifold, which also means the lambada sensor and the heat shield.
The lower heat shield nuts will likely snap off but the others will come off okay.
*You'll need some M6 nuts and short bolts to patch up the snapped ones, otherwise the shield will rattle.
My lambda was welded (rusted) to the manifold and did not want to come out without the hex nut becoming deformed.. I had to put a new lambo in it... So Plusgas/Release Spray yours to death before you start.
*You will need a new exhaust manifold gasket.
Some of the manifold nuts will be rusted to the studs and will unscrew the studs from the block rather than coming off the studs... but no worries... remove the nut from the stud off the car, clean up the stud and then fit a new flanged nut to it afterwards.
*You'll need a stud socket to screw in the stud.
*You'll need some M6 flanged nuts. Missus!
You'll have to separate the exhaust flexi' from the manifold..
*You will likely need a new U clip (52-55mm from memory?)
...and you will discover that the clamp that braces the manifold against the block is rusted to buggeration.
*Search for "Fiat 500 Exhaust Clamp 51816520"
There is a brace bar above the exhuast.. just 2 or 3 bolts to undo...
Drain the oil.
I nearly forgot.
I can't remember if one of the sump end bolts is obscured by the bell-housing/flywheel cover plate... (10mm) but with everything out of the way, you "just" need to undo the sump. Make sure you have removed the same number of bolts that your new sump has... then it should just drop down..
In reality, you'll need to wrestle it. Don't stick a knife, screwdriver etc. into the mating surfaces in case you nick it... but you will need to apply some force. The old sump on mine did actually get deformed around the rim, from being prised/levered off, before it separated. Just take your time, tapping it left and right, back and forward etc... eventually it will come off. You just need to move it sideways to clear the pick up.
Clean up the mating surfaces with a wooden lolly stick, WD40, Contact cleaner, petrol or anything that will shift the old gasket goo.
Let it all dry.
Stick half a tube of gasket goo onto your new sump and onto the engine block..... (you can't use too much, IMO) and then refit the sump carefully, making sure not to knock the goo off, particularly at the curved ends. I put extra good at the curves.. on the block as well as on the sump.
Refit everything in reverse order. Robert being your mother's brother.
Ralf S.