I'm currently searching for a suitable bike (or 2), as I feel strongly in favour of this class of racing. Over the last few years, current VMX bike costs have severely stretched the budget, and I'll probably be broke when the VB reaches the" looks just right " stage.
I think a SL100/125, DT100 or ts100, though I have a friend who used to race a Bridgestone 90 with a 100 barrel and noisy pipe which could run with any 125 in pre 70. Pity it has a pressed steel frame/swing arm which tried to bend into a U shape each meeting.
Bultaco used to have a Lobito 100 model scrambler which came with or without lights, Tiny little thing, went quick, and handled very nicely. Definately nothing like the other lobito turtles! It might fit the current guidelines, but not sure the spirit of the class.