Yes, you can use the stock cam with carbs.
If you wanted to upgrade on the cheap, you can try to find a cam off an RD motor (86-87 1.8l 8v gti/gli).
Otherwise, there's the 268* cam that should give you the power it should have had all along.
If you don't mind upgrading the valvetrain, the 276* is highly recommended, especially with carbs if you don't mind setting the idle a little higher and having a slight lump. The power is great from what I hear.
As far as cabriolets go, there wasn't one based on the A2 platform.
What year is the cabrio? I'm assuming later on with digifant / hydraulic lifter head?
For the Fox, a nice upgrade would be sourcing a dual outlet manifold and corresponding downpipe (custom if need be). It's really necessary to open up the higher revs.