Stick a drum brake wheel on the back and a std front end and go EVO and Pre85.
That's where I'd put it. There's a big vibe out there for these bikes that
could end before it starts if rules and era's for the vintage framers aren't addressed. I understand and support what SON is doing in promoting these bikes but I believe that we need to keep the classifications loosely in line with MoMs. To simplify the whole deal I reckon there should be two loose vintage age cutoffs....pre 75 and Evo. Bikes like the above Yamaha would fit into Evo while machines like my little 360 Yamaha short tracker (
below)would slot into pre 75. I think the bikes should be classified under speedway regulations with their own section if they're being used for long track or short track
but be classified under Classic Dirt Track if they're being used for traditional short circuit/dirt track. Period disc brakes should be allowed for both classifications with no front brake for long/short track and a mandatory front brake for dirt track making it easy to swap from one discipline to the other by just changing front wheels....basically the difference between Dirt Track and TT racing setup in the US.
For people to become interested in buying and/or importing these type of bikes they need to be capable of being used for both types of racing to get more bang for your buck. To make that possible, some rules, most specifically the drum brake only criteria for pre 75 and Evo need to be repealed more sooner than later.
Below: My MoMs legal pre 75