Hi all !
After drooling for the last years over those wonderful Konas of yours, I am now part of the Kona "chapel" since I recently found a nice and affordable Cinder Cone in my size. It's a 1994 orange one fitted with a Marzocchi XC51, the same as in the Konaretro catalog.
I plan to change that fork for a 2009 RS Recon SL 80mm because I want to use it without reservation. So I'd like to have your advice on the subject : is it a sensible choice or will the bike handling turn weird ?
I don't think it would turn weird, but it is wise to match the bar/stem combination to the head angle. In other words, making the head angle slacker has a similar effect to fitting a long stem - both will make the steering feel less lively, the bike will be more reluctant to turn. So to counteract the effect of the slacker head angle, you should consider fitting a shorter stem to liven the steering up again.
Also bear in mind that the question is how far your hands are in front of the steerer, looking down the steerer (i.e., not horizontally). So that means that a stem with rise is in effect slightly 'shorter' than a zero-rise stem of the same length. It also means that fitting bars with more sweep has the same effect as fitting a shorter stem.
This can have a powerful effect, so there's no need to over-do it. e.g., a 60/10 stem combined with swept risers would do far more than compensate for the longer fork and would give ultra-lively steering. It is a matter of personal taste really where you strike the balance.