Of a "light motorcycle" is one of 75-125cc with an output of no more than 11kW so unless you fit a big bore kit to it it's not going to change.
As you've de-restricted it and want to keep it legal you're going to need to inform your insurance company. There's a very high likelihood when you do so they'll no longer wish to insure you so be prepared for that! You don't say wether you're on "L" plates or have passed your test. I can't really give you any more specific info unless you say why you wish to ride a de-restricted moped rather than a larger capacity machine (I'm guessing finances may have a part to play?!). Try the website below for a little more info on bike categories...
To change your bike to a light M/C would likely to be impractical, i.e. the amount you would have to spend both financially & in time would more than cancel out any benefit you'd get from the modifications! You'd be better off just buying a bigger bike.
Happy Riding tho!
If History has taught us One thing....
It's We've learnt None of Histories Lessons!