Hi Alex,
In my humble opinion I don't think it would be worth the trouble to modify the diaphragm especially if it involved heat to modify the shaft as it could damage the diaphragm.
Might be Best to try to return it or cut your losses and find the correct one.
I have attached a link to a site in Australia that sells the correct diaphragm but I am sure they are readily available in Europe. The main reason for the link is that it shows all applicable vehicles. This may aid in the search for a correct one.
https://classiccarbs.com.au/index.ph...ines-900.301D-
Hope this is of some help,
Cheers,