The first thing that comes to mind is rather than buying a $2000 crankshaft, you could just go to an engine/crankshaft balancer and have your existing one balanced.
I've never had it done myself, but I have a mate with a highly strung 4AGE that went down this road. I'm pretty sure he used Paul England- but don't quote me.
Dynamic Balancing | Paul England Engineering Melbourne
Maybe give them a ring and see if what they offer suits your needs. You'll probably have some reputable engine balancers in NZ that you can call too.
A couple NZ balancers off the top of my head:
Franklin Engineering
Franklin Engineering - Franklin balancing specialists
ATL Balancing
Home @ atlbalancing.co.nz
That said with the right cams, ARP bolts, HD valve springs etc etc 2TG's are known to be quite rev happy and will rev to 8500 and beyond with (as i understood) stock cranks.
Other than that my only recommendation woud be to look through the forums to see what other toymodders have done, there's more than a few very worked 2TG's on here that would serve as good guides for what you're planning to do.
Cheers
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