ok, people. i just wanted to share with you the solutions i've found in order to be able to enlarge.
it turned out impossible to find anything with the same mount my Belar lens had. all i could find was used ones, with nasty pictures of it... no way!
it was very easy to find somebody to make the same plate but with a l39 hole in the middle, so i've done it! right now there's a russian industar 50 on its way to my home.
at the same time, yellowcat made me this offer for his schneider componon, and all i had to do was to take the old plate for the same guy who made me the l39 and ask him to open the hole a little bit wider. now i'll have the 80 and the russian 50!
i'll attach some pictures.
i want to thank you very much for your concerns and help. it was very useful for me!