Hello & welcome to TRX250R.Net fastest250r,
You can contact BVC Racing for a price on a new top end kit.
Sence your going to buy a top end kit & not knowing if your wanting a new or used one, if you buy new, it would be best to wait till it needs a bore job before re-porting the cylinder.
If you buy used & it needs a bore job, then the re-porting could be done right away before a new piston is fitted.
If you buy a new unported one, still, it would need to wait till it needs a bore job before any port work is done.
Sence your from atvriders, you've seen the one for sale on there & that might be your best bet for a used non p/v top end.
We have a member on here with a p/v 310 for sale that already has my port work.
Re-porting prices start at 250.00 & will be a pro port job like a sponcered pro rider would need.