Recreate an smooth editable step file from 3d scan and STL

I have a basic 3d scan of a highly curved object that I need to get recreated into an edible STEP file - I'm at the limit of my software and don't want to give them any more money! I can create faceted files from this, but they're useless. I'm pretty sure this is a quick job in something like Fusion which I can then tweak once the shape is splined out hence low price, but happy to be told otherwise. I just need it simple, smooth and tweak able (the curves fit into a vehicle).

I'll then take this top and fuse it with the base I have designed. Screenshot attached. Any questions please ask. Could be more collaborative work in future up for grabs as I only specialise in certain 3d design.