How big is the hole and can you still get at it without pulling what you currently have built apart?
If you can get at the hole from behind, thats from the side of the piece that will be facing inside the model, cement a peice of sheet styrene slightly bigger than the hole over the opening with CA glue. Once the piece of styrene is secure, flip your kit part over to the side that will be the visible part and fill in the hole with epoxy putty,I recomend Milliput superfine (thats the white coloured one). The styrene piece will act as a backing for the putty and when all is cured, you can sand the Milliput flush with the rest of the surface and scribe correct panel lines into it.
If you cant get at the hole to patch from behind, mark a border around the perimeter of the hole, about 1/8 inch further out from the edge of the hole in all directions, and carefully use a small file to grind down the marked area slightly,
cut a peice of fine, stiff metal mesh to the shape of the entire area so that when its glued down, it sits with its edges on the filed out area. The mesh should sit slightly below the surface of the plastic and act as a support for Milliput over top of it. Again, once the miliput is cured, sand it flush with the model surface and engrave correct panel lines.
I know that sounds like a lot, but believe me, this is one of those rare occaisions when something is easier done than said.
I hope it works out for you