You should be able to get away with a 65mm travel fork like an old Bomber Z2 or a SID with a short-ish stem. I really think 120mm would be too much I'm afraid.
Thanks for the comments guys. I think I'm going to sell the bike and build a new HT. I love the Kona and sorced some cool bits for it like the USE stuff but it sits in the shed and never gets used.