There aren't many 91-92 hybrids because of space limitations presented by the rear shock.  The intake needs routed around it to fit. 

I've ridden multiple Zillas and a few too fiddys.

I originally got a 91 LT250R.  Wasn't enough power so I got a 500 engine and a 88' frame.  Swapped over everything, but the rear suspension and engine.  The power increase was awesome and that was with a basically stock 500 engine.  The 250 frame handles much better than a zilla.  The zilla is a large ATV and feels bulky when compared to the 250.  The 60-90lbs difference between a hybrid and a zilla is noticeable too. 

You won't notice the weight difference from swapping out the 250 engine for a 500.  I think the weight difference between the two engines is less than 10lbs.