Actually, an OEM 1G tank bra WILL fit a 2G bike... but a 2G won't fit a 1G (it'll be WAY too loose). I had an OEM 1G cover on my 2G for >6 years. I think the dealer had installed it thinking there was no difference and Suzuki hadn't come out with a 2G cover yet in early '01, so that's all there was. Only reason I changed it was because my son wanted a tank cover for his 1G and couldn't find one, so he bought a 2G cover and we traded.
The seams won't line up perfectly (but then the correct one isn't perfect either). It will also be a tight fit and the bottom edge will leave a fraction of the tank uncovered. Frankly, I liked the fit of the 1G cover on my bike bettter than the correct one because it fit so snugly.