Modern Vespa

I have replaced my Fly 150 rollers with different weight ones along with a belt change.

My question is do new rollers and belts have a break-in period? I seem now to have less power than I did before.

Any ideas?

Nope, there isn't a break in period for either the rollers or the belt. Either the weight of the new rollers is the problem or you didn't seat the belt all the way down in the rear pulley.

Did you use a Piaggio replacement belt or a cheaper aftermarket one?
If you did use another brand no telling what's up with it.

You can't just arbitrarily change the weight of your rollers -- without knowing exactly what you are doing and why -- and not have the likelihood or either loss of acceleration or top speed. With some experimentation with subtle weight changes (half a gram less or so), you may be able to get a small improvement in acceleration without sacrificing top speed, particularly if you are heavier than average, but "shot-in-the-dark" changes in weights are more likely than not to have negative consequences.

Also, if the new weights are not precisely the same size, your performance will definitely suffer.