There is one typo here, dear Francky.
"Visita interiora terrae rectificando
invenies occultum lapidem" [no double "n"]
Actually we have two verbs here:
"visita" - imperative from "visitare"
"invenies" - futurum from "invenire" just like you suggested above.
"Rectificando" - gerund from "rectifico, rectificare, rectificavi, rectificatus" - rectify, regulate...
Bridge:
"Visit the depths of the earth, through/by purification (or: by purificating yourself), (where) you will find the hidden stone"
or like others translate:
"Visit the interior of the Earth, in rectifying you will discover the hidden stone"