The bolt you remove from inside the clutch cover needs to be replaced by the standard length (shorter) bolt to allow the oil to circulate internally
Put the banjo and blanking bolts that you take off (along with their copper washers) in a plastic bag and tie it to the cooler. It's a valuable part - worth more with all its correct bolts I am surprised that you haven't had a flurry of messages asking you to sell it.....
Lots of info here - https://ttr250.activeboard.com/t44874914/oil-cooler-information/
i would get the short pipe before you remove the kooler...
I put a bigger 5 pass kooler on my bike with a short header
pipe..had a bit of room to spare..