Thankfully they're different engines so that one's nipped in the bud, but what about Yamahas where the MX bits bolt right on?
I dunno how much it matters, really. If we were talking 125s, a DT125 barrel is a better starting point than an MX125 item - and its dead easy to simply copy the MX porting specs straight into a DT barrel anyhow (assuming you thought that was a good idea, which it isn't). I assume that the DT and MX100 barrels are the same story.
So unless you also introduce a rule to prevent custom porting, then who cares whether its a DT or an MX barrel? Such a rule would be very hard to enforce.
I guess the LT2MX would fit into the same boat as the TS90MX - but as they were essentially kit bikes, it'd be hard to keep all of their bits off their 'standard' cousins.
I like the AHRMA idea of publishing a list of bikes that you CAN use. If it ain't on the list, you can't use it.
Here's the small list I started with.
Suzuki:: TS/TC 90, TS/TC100
Yamaha:: DT100, LT1, LT2, DT/GT80, AG100*
Honda:: SL100/125, CT90/110, XL100/125, XL75
Kawasaki::G3/G4/G5 100
Jawa 90...........
Hodakas?
Geminis?
Also..Do you think that making the cutoff pre'78 allows for too much of a development spread. One punter reckons pre '75 would be better. I'm not familiar with the product range to offer a serious opinion.
What ideas have you got to contribute.............................
Can't speak with any knowledge of the other brands, but:
A non-bubble-tank XL100 is definitely a nicer bike than the older SL/bubble tank XL100. Does that make it a faster MX bike? I don't know, but I suspect that the difference will be marginal.
In Yamaha land, the 74 and 77 model DT100s are basically identical, and the DT100 is heaps better than the LT1/2, so (provided you allow model run-ons) there's no difference whether its pre-75 or pre-78.
*This might sound like a weird addition, but the AG100 is basically the same bike as an LT2, but are much easier to get bits for.