Hi Paul hope you're doing fine as well, when you turn the ignition on you should hear the fuel pump priming as it's located underneath the rear seat, if you don't hear the priming then the fuel pump is probably at fault, SPI & MPI are different pumps though due to the fuel pressure so if you put an SPI pump into an MPI or vice versa it'll either not run or not run properly, your brother's Seicento is an MPI being a 52 reg. Usually though Seicento fuel pumps will give you a warning that their failing with a constant very high pitch whining noise before finally giving up.
Although what you are describing my first & probably only suspect would be the crankshaft sensor as on both Seicento's & Cinquecento's it's a very common issue on both 899 & 1108 engines as the sensor will either be covered in muck, the sensor bit itself might be cracked as it has been known to happen or there will be a break somewhere the wire & they do just go without warning, It'll be located somewhere around the crank pulley, be sure to get a reputable manufacturer part as the cheaper ones are not so good. Magnetti Marelli will be the best as that would be what was originally used but any decent brand should be good enough & they come with the wire as part of the sensor as OEM, it'll usually be around the £20-£30 mark so much cheaper than the fuel pump. If it's on a bracket like the 899 be careful not to bend the bracket in any way as it can alter the timing & signals to the ECU as there are tolerances for firing, fuel etc. Again it seems that SPI & MPI are different but if you buy from either Shop4Parts or Ricambio it should tell you on their site.
Below is the sensor of my 899 with the bracket on the block when I had minor running/starting issues last summer, although in a different position to the 1108 the principle is virtually the same, luckily I was able to clean this one up & reuse it as the wire was intact & the sensor wasn't cracked although. As yours isn't running at all I'd suspect the wire being broke or sensor cracked more than it being grubby. Hope this helps as it's the only part really that would stop a Seicento running apart from a pump as they are pretty basic & robust little cars but if that's priming all good then it'll most likely be the crank sensor.
Although what you are describing my first & probably only suspect would be the crankshaft sensor as on both Seicento's & Cinquecento's it's a very common issue on both 899 & 1108 engines as the sensor will either be covered in muck, the sensor bit itself might be cracked as it has been known to happen or there will be a break somewhere the wire & they do just go without warning, It'll be located somewhere around the crank pulley, be sure to get a reputable manufacturer part as the cheaper ones are not so good. Magnetti Marelli will be the best as that would be what was originally used but any decent brand should be good enough & they come with the wire as part of the sensor as OEM, it'll usually be around the £20-£30 mark so much cheaper than the fuel pump. If it's on a bracket like the 899 be careful not to bend the bracket in any way as it can alter the timing & signals to the ECU as there are tolerances for firing, fuel etc. Again it seems that SPI & MPI are different but if you buy from either Shop4Parts or Ricambio it should tell you on their site.
Below is the sensor of my 899 with the bracket on the block when I had minor running/starting issues last summer, although in a different position to the 1108 the principle is virtually the same, luckily I was able to clean this one up & reuse it as the wire was intact & the sensor wasn't cracked although. As yours isn't running at all I'd suspect the wire being broke or sensor cracked more than it being grubby. Hope this helps as it's the only part really that would stop a Seicento running apart from a pump as they are pretty basic & robust little cars but if that's priming all good then it'll most likely be the crank sensor.
The sender & fuel pump are all one piece so to replace the sender you'd have to get the whole pump. The tank doesn't need to be dropped to change the pump/sender unit, all you have to do is lift up the base of the rear seats which will reveal a cover, remove this cover which is held in with 4 x 10mm nuts from memory which will then reveal the top of the pump & tank, remove the connector & the 2 pipes which go into the top of the pump taking note which one goes where & also expect a little fuel spillage from the pipes, then there will be a metal ring around the pump with I think 10 x 8mm nuts from memory which need to be removed as it's the securing ring for the pump, once the securing ring has been lifted off twist the pump whilst it is still in situ & then lift it out of the tank.
. As for the oil leaks between head & block only thing I could think of is possibly the rocker cover gasket leaking & the oils running down from the top, rocker cover gaskets are notorious on FIRE engines.
. Cento senders/gauges all tend to do their own thing & are very rarely accurate it's a well known thing with them 
