Zenith's are getting a bit scarce and the copies are of hit and miss quality.
Fuel consumption is a little better with a Weber as they run leaner but at the expense of oomph and are said to be more prone to jet blockage.
It's all a bit swings and roundabouts. If you're happy with how it runs I'd leave it.
A 2.5 carb would be nice but there's a fair bit of faffing, manifold/downpipe/cable to sort out.