That a pretty easy fix...other than draining the cooling system. Draining is easy, but refilling and bleeding can be a nightmare!
Easy to get to from the top:
O-rings seal these 4 ports. We had a bad oil cooler that was mixing oil with the coolant, luckily not the other way around.
New cooler installed we bought the "S" version which is a bit bigger and fits right in.
Good luck!!