The 1980 cut-off worked fine, it even stopped the alloy Spondon frames now entering the odd race here & there. So what happened to the 'no alloy frame rule'.
There are quite a lot of small manufacturers out there making almost everything for the TZ350's now, don't spoil it.
If you are unhappy with your RZ make it a 421cc non-power valve, or a 485cc banshee/TSS (Cairns) and race it in P6 500 class.
Don't even mention the unlimited 2valve vs 4valve situation.