FWIW the KTM shafts and gears are different diameters splines etc
the one most likely to be the same is the 2000 husqy SM610R200 but ive not measured one
Im not sure what the shaft spacing is on the KTM or the husqy but on ours its 60mm
as far as I know all years of berg have the same basic gearbox layout
if you could find a gearbox with the same shaft spacing it would be relatively easy to adapt as all the 6 speeds ive ever seen have the same shift pattern. check out the new KTM boxes that have sliding shift rings instead of sliding gears
FWIW
I have a Hayabusa gearbox and a kawasaki ninja 650 gearbox waiting for me to modify my cases to.
the busa box of course is too wide need to lose 1st and second gears to make it fit and shaft spacing is 69mm so the cases need modifying, its doable if i got to 2 separate radiators and make the engine longer, these gearboxes handle 300+ HP so its my ideal base for a "new" 4spd sand bike engine. they measure 54HRC hardness
the Kawasaki ninja gearbox was only $150
only requires a slight reduction in width of the first gear pinion to fit and its shaft spacing is 65.2mm, measures 60HRC
most bikes (these 2 included) have a 25mm diameter input shaft so clutches are interchangeable and being current model jap bikes the parts will be available for a long time.
will post pics when it happens .. I promise