RJM, can't tell in your first pic what type hub was used to mount the original go kart flat track tire/wheel assembly, but if your going to use the ATC70 wheel assembly they are 3 on 100 bolt pattern. You should be able to use a ATC90/110 '85 thru '80 axle and tube spacers assembled with your hub and would install onto your forks no other mods needed.
"HJ"
’84 YT60
’85 350x (423cc) with ’81 XL500 head conversion
’83 XR500 (628cc) converted 3 Wheel hybrid
’73 CB750 K3 converted 3 Wheel hybrid
’86 TRX250R chassis XR650L engine hybrid
’89 TRX250R (stock )
’90 LT500 Quadzilla
’85 FL350 Odysses ( water cooled head )
Lost (sold) but not forgotten ’80 ATC 110 (205cc on alcohol ) ’83 200x ( 222cc ) ’83 200x ( 185s eng ) ’85 350x ( stock )
added to the collection recently, (2) '70 US90 Aquarius Blue and '74 ATC90 Daytona Orange