There is both the radiator and an oil cooler in that area.
The best bet is to find the corresponding area on the bitmap, and paint it like the other sections of fuselage.
Well to be honest sir there is the limiting of the graphic sheet. Not enough pixels for that area your not going to get the same. Your would need a lot high res skin for that. For instance in DCS P-51 you get a about ten 2048x2048 pixel sheets and the aircaft parts are split up over the sheet. The P-51 in AH is on 1 sheet at 1024x1024. Now you see the limitations of the graphics in Aces High skins.
Here's my take on the radiator internals....
Looks good. Nice work.