I'm deleting what I wrote before because I stupidly thought you were talking about turning a Drake II into a 1.6, not turning a Drake II 1G (CST454CUFG) into a 1.28. I just didn't read carefully enough.
The guts are different, and I don't know what if any adjustments can be made on the 1G flush thingy.
I suppose you could rip out all the guts and put in a Korky 3" Universal Flush valve and a regular trip handle, but I wouldn't count on it unless I measured the hole in the tank. Not an easy fix, though.
Sorry I didn't read carefully enough.
One thing I am curious about is ChefWong's impression of the 1G as "petite". From the photos, it looks as if the regular Drake II is sleeker than the 1G. The tank on the 1G has always looked boxy to me in photos, particularly when compared to the regular Drake II. That's why it's important to see this stuff in person, because apparently the tank makes a different impression; could be it is smaller than it appears, something you can't tell without something to compare it to.