What could cause extremely worn rocker arms?

[WastelandParadise]

Hi all,
This is my first post here but I love these little bikes. I recently went from New Orleans to Seattle and back on an 01 GZ and she took it like a champ. I'll post pictures if anyone is interested- the whole thing took me about 2 months.
My girlfriend is getting into riding and we picked her up a 08 GZ250 with a pretty nasty ticking issue... got into the head and it would seem that the rocker arms are WORN. REALLY worn. I've attached some pics for reference. Not only this, but the exhaust rocker arm is loose on its shaft. This is pretty unbelievable considering her bike has less that 2000 miles...
I've read and experienced the heads being the weakest point of the bike, but 2000 miles should not have put this much wear and tear on those arms. I'm thinking its oil starvation but I'd be interested to see what y'all think. How would one test to see if the oil is getting to all the right places? Turn the bike on without the valve cover and let it fly? Serious question, I'm just not sure.
Next, is it possible to put 01 GN250 rocker arms in the GZ? The part numbers are different on partzilla which tells me no, but my intuition says otherwise. I've replaced my jug, piston and rings with GN parts with absolutely no hickups despite them being listed as different part numbers on partzilla. It would be ideal to use GN parts where possible because they're possible to find and a fraction of the cost.
Any input is appreciated y'all, TIA!!! Cheers!!