Cmon, guys. Isn't our sport mostly about fun?
I personally would not know what arm is allowed on what bike & don't particularly care.
I would not know the difference riding with Moly versus Alloy swing arm & I bet 99% of other riders wouldn't either.
To the spirit of the rules sums it up. We (Viper) use to let YZ125G (1980) run in the pre 80 125 class even though they were a big step up from the E & F.
Didn't care. Used to do the same for the 1980 RM125. Again, didn't care. (Yes, I rode that class) (Used to get hosed by a guy on a 73 CR)
I suppose for Nationals & the like we need a "Line in the sand" but for interclub, like Viper for example but I am not speaking for them, some "Own Rules" could apply.
My view, if it doesn't give a huge advantage, let it go. Alloy arm looks better but could most of us tell the difference? I think not.
Leaving Evo aside, if it was available for that model, OK then. Not talking about what works bikes had on them, what was available to production bikes. Did someone in the day put Yamaha forks on a RM? Possibly but why?
We let people go overboard with engine internals, fork mods (emulators), suspension kits (Race tech valving etc.), new wheels & spokes (stronger & lighter than OEM) etc. etc. etc.
I realsie lots of stuff is no longer available & it would be stupid to stop someone riding just because they had a 2013 clutch lever & I also realsie this is allowed for as a incidental part & can be used.
What is to stop someone buying a 1975 lathe & milling machine & making lots of stuff as I'm sure some did. As I said, spirit of the rules", & line in the sand. Weren't DT's air cooled, drum brake etc. till 1991?