maywong, I wouldn't mind seeing that picture if you don't mind? I love seeing different doll hybrids.
Gift_in_Edge Hmnn, the price points on Japanese Ken and G.I. Joe Zero Pilot seem to be nearly the same from where I'm looking (Amazon for Ken and Ebay for Joe). I think the G.I. Joe is a bit....plain looking to be Kato. It's more that the figure doesn't quite fit Kato's more slender appearance and his features don't really scream "Asian" to me, and if anything he looks a bit more Chinese in my opinion (nothing against him, just for this specific character I'm not seeing it). That's why I thought of Japanese Ken to begin with, because he's got Asian features that are fairly prominent, and he's a bit less overtly masculine without being outright feminine, like Kato is. Plus, for consistency's sake with Britt and his plastic hair it would be easy to take out Ken's hair and give him an apoxie sculpt haircut in Kato's exact style.
maywong's idea of a generic Ken head repaint is just as good, it just really depends on how far you want to take it with the character and him looking aesthetically Asian. A regular Ken can be repainted to have Asian eyes, but the nose, cheeks, jawline, etc, are still going to be rather your general Caucasian look (Yes I know Barbie/Ken comes in all shapes and sizes but generally Ken has a very Caucasian aesthetic unless you search out specific sculpts, some of which are more rare than Japanese Ken by far!) unless you can modify the head that much.
Again, this is all just suggestion. I feel like I'm coming off bossy in this post and I really don't mean to. He's your custom, your Kato, and how you want him is how you want him!!
Please forgive me if I sound know-it-all like. I'm far from an expert, just suggesting would I would do. I mean it as advice only. ^^;