for such sealing a cheap and good product is sillicone rated for up to 300 degrees. It usually comes in red or in black color. Just google silicone 300 degree in your language or ask in any store which sells parts for car or building materials for houses. and dont think of buying the one for 1000 or 1200 degree that one turnes into stone and is only used in houses around fireplaces. vibrations makes it crumble to dust. the one that is rated to 300 degree stays nice elastic for ever and it works for your hot temperaturs. its also used to seal thermostat and water pump if they dont come with proper gasket.
In your radiator case - dont tuch it. No you can not put normal bleeding screw back even if you find one because the original hole for it is nice small, in your case there is something big in place where it used to be and it might be that the original little hole doesnt exist under this big black thing… From the picture i cant even tell what this big black thing is. Is it hex shaped? Is is such big screw? Is it such big cap? Just random piece of plastic glued over the where the initial bleeder screw was? I cant tell,.. from what i know about punto 188, mk2, there was always way more smaller plastic screw in this place. Screwed into yellow plastic, more detailed position: right under the big black thing. You can try take the black thing away to see whats under it and then if no other option use the above mentioned silicone to seal it back. And if you still want to repair it properly i would suggest to replace the radiator entierly, new one comes with the desired screw – dont worry – you maybe will want to add a little grease to its thread for it to turn a bit easier by hand.