Has anyone tried to modify one of those large scale plastic WWII diecasts.
I have a 1/18 scale Corsair from www.updiddilyupup.com
I would like to add a motor to make the prop turn slowly and maybe some nav. lights or some thing.

Just because I'm a horrible nitpicker, I'm gonna point out it's not a die cast - die cast refers to metal toys. These things are big plastic toys, molded like any other plsatic thing. :)

I have the P-47. I was going to repaint it in my Dad's markings and accurize it some day. Looks like it'll take an awful lot of work to make it look like anything other than a big toy, though!

technically you're right.
I think of diecasts as anything prepainted and put together and a model as something I get to paint and build.
Which brand of P-47 did you get?
updiddilyupup.com has a Corsair and a P-51 from BBI which have greater detail, both built and painted than 21st century although 21st has a much greater selection and I think they're getting better.
They are soon to release a "pappy" boyington corsair and an avenger is in the works.

I got the 21st century P-47D razorback from Mad Cat Aviation. It's hanging from the ceiling over my head right now :).

Picked up one of the smaller P-40 Flying Tigers. Unfortunately the profile looks more like a P-47 because of the bulging tunnel that runs between the wingroots, so I'm going to replace the wing with a Revell P-40 wing section. Also going to vacform an accurate windscreen and canopy - the toy has a bubbled canopy.

I like 21st century's 1/32 scale series, they've just come out with quite a few more.
I have some on my web site and have the Me 262 personally. Its just easier when you have kids to get a finished product and do some light modifications.
Post some pics of your completed P-40, it sounds interesting.