If you have a 20" frame you actually have quite a few different options regarding fork choice as the longer wheelbase of the larger frame is less effected by different axle to crown heights.
I have a 20" 97 King Kahuna (geo is the same as almost all other 97 Kona hardtails) and am running a 100mm Z1 on the front and it works fine for me, but it probably as long as you'd want to go.
Others have happily run 100mm Fox fork on the 20" frame too.
I tried a 65mm Marzocchi Superfly fork on my frame and found that while it climbed great, it was pretty sketchy in the tight stuff and downhill.
In my opinion I'd say an 80mm Marzocchi is the perfect fork for the bike for all round riding, but your choice will be infulenced by your riding style, the type or riding you do (and where), and what else you are currently riding.
I hope thats of some use.