I've never heard of anyone making and installing a lip on a G50. I know there are a few atermarket body kits out there, but a lot of them are, in my opinion, hideous.
IMPUL makes (made?) a kit for the G50, which I imagine is something along the lines of what you're looking for, as far as the front lip goes. Unfortunately it looks like their page for the G50 is in Japanese only, but the pictures will hopefully tell you what you need to know:
http://www.impul.co.jp/products/complete/g50.q45.htm
As far as differences between the early and late G50 bumpers go, I think they are pretty siimilar as how far they go down. Here's a side-by side shot of my '94 sitting next to a '91:

Both bumpers seem to go down about the same distance. It's a bit hard to tell from the difference in ride height, but they look pretty similar. The late bumper slopes down more on the top surface, but there is less vertical flat space between the tail lights and where the top surface of the bumper starts.
This shot was taken before I had completed my dual exhaust. It has chrome tips on it now and I hid the hangers a bit better.