That is one freaking strange Mo-Fo...
The engine type number stamped on the block means your engine started life as a 899cc motore.
The bore is 57.8 now so it has clearly been sleeved down (you can see a thick liner). It would be interesting to know what the "bore" would be if you measured the liner as well. The standard 899cc bore was 65mm.. and when I measured your bore on my screen and multiplied that measurement to be 58mm.. then my measurement of your bore plus your liner with the same muliplier comes out at 66mm (+/- for some distortion...) so it looks like your new liner was just squeezed into your old liner - although the old liner may have been machined out a bit first.
At the moment your motor (if the stroke is standard) is 710cc.
It's nothing to do with the 126 650cc or Polish 704cc Cinquecento engine, since that was a twin cylinder engine and they had a bore of ~73-80mm. The pistons would be huge compared to yours.
Taking a step back.. why are you fixing it? If you want to use it in your car, then the questions to answer are;
1) Do you want a 710cc or an 899cc engine, bearing in mind that the 710 is not standard
2) If you want to continue, what will you do if find other issues or problems (like where can you find some new piston rings for these "unknown" pistons)?
If it was me, I'd send that engine back to the seller, or if it's not viable, keep it as a hobby but try to find another engine that is still standard and 899cc for your car. There must be plenty still around in breakers yards, for not much money.
** Meanwhile, what's stopping you from pulling the pistons out (with the con-rod attached) from the bottom of the block. If you press down on the piston crowns (especially 2 and 3, which are in the middle of the block) why don't they come out of the bottom?
If you can get the pistons out, you'll have a chance to save the block - i.e. knock out those liners and fit standard ones, so you can make the beast an 899 again... although the parts might cost more than a whole second-hand engine.
Ralf S.