A '99 model petrol will be a rare beast these days. Worth looking after the very early ones, especially if in good condition.
I bought new interior handles (the big part, with the loop that comes up above window line, not the little handles that open the doors) a few years ago to replace the originals, because they were being degraded by UV. The surface of the plastic literally starts to crumble. Is that the kind of damage you mean?
It would be worth checking with a Fiat dealer to see if they are still available. I'd wager that there won't be many used ones around that aren't starting to suffer with UV degradation, so trying to find new ones might be the best option. I have a funny feeling though that they went west when Fiat Europe had a big clear out of spare parts for the Multipla back in 2015/16.
Those handle panels are moulded by an unusual process. I remember looking at them when I replaced mine an being a bit confused as to how they'd been made (moulding design is part of my job, so I get a bit anal about such things at times

). Some of the wall sections are very thick - far thicker than a normal injection moulding. The Multipla is littered with such manufacturing novelties. That's. Part of the reason I love it. Take a look at the door window frames, for example. They're very skinny, but made of steel, so unlikely to be an extrusion. So how were they made?
Anyway, back to the door handles: Now that 3D printer build envelopes are starting to grow, they might be a good candidate for replicating by that means, especially the ones on the rear doors which are quite a bit smaller than the fronts. Or it may be possible to make some moulds from a cleaned up original, but I'm not sure what material would be suitable for the part itself. Some sort of glass filled resin, perhaps.