If your 620 has a 20mm front axle like other KTMs and Husabergs in 2001, then just order KTM part #SXS03540400. It's an adapter kit that allows the old wheels to fit on the new bikes. I have it right in front of me here. It comes with 2 sleeves that reduce the fork holes down to fit the old axle, and a new left side wheel spacer with a larger shoulder on it.
I haven't tried this kit out yet, but I'm planning to use it to mount my 2002 Berg front wheels on a 2008 KTM 250sxf when my broken wrist heals.
KTM used to make a kit to go the other way (new wheels on the old bikes), but I think it has been discontinued.
