I cannot give you the answer of what would be a good cooler to replace the radiator installed oil cooler.
Reading your post your state that you are worried your oil cooler is dirty. This indicates to me that you "May" have not removed the radiator/oil cooler and sent it to a good radiator shop for cleaning and testing.
If you have not cleaned/tested the radiator, remove it immediately and take it to a good radiator shop. This will solve your concern about a "dirty" oil cooler and give your new engine/radiator the best possible heat transfer capability available.
In "my" opinion, it is a major MISTAKE to spend all that money on a new engine and NOT install a new radiator/oil cooler or have your current radiator/oil cooler cleaned and tested.
I hope this helps.