Search eBay for Campy freewheel hubs. Then you can put a 7-speed freewheel cassette on there.

For 8-speed or higher, you'll need a Campy freehub and they are 130mm. While you can easily wedge the rear wheel in there, it's tight. Better to coldset the rear dropouts to 130mm.

You can find Athena components on eBay; I've got Athena brake levers and a derailleur. I recently saw an Athena crankset.

Yes, there are newer components that will also fit. Pretty much everything. I'm using my Athena stuff with some older(90's) Campagnolo Chorus stuff to build up a bike. I'll be using an 8-speed rear hub with some new rims.

I'm guessing the Basso has 27" wheels so you'll have to hunt around for 27" rims but there are a lot of them still around, even new ones from Sun, Weinmann and Velocity.