20mm needed?? it will take to much time and money to fix the situation with this fork, plus you might end up always worrying about how it will hold up on the road. Just go get a correct sized fork for the bike and cut your losses. You can always sell the too small fork to help pay for the proper sized replacement.
JMOs
Chombi
84 Peugeot PSV
85(?) Vitus Plus Carbone 7