That mast float may not be 'big enough' if it's the standard Hobie Wave or Getaway float. The H21 mast (and everything else) is a lot heavier, with more leverage and will probably just push that small float right under. And a bigger float will add a lot of drag way up high, where you least want it, when you're right side up.

You might consider putting some type of water activated inflatable CO2 cartridge type float up there, that way it's not much drag when you are sailing, and it only 'blows up' when you need it, ie. when you flip. Sort of like those new inflatable life preservers being worn by offshore racing crews, just wrap it around the top of the mast or attach with bungies, etc.

On the big sail, on spinnaker cats, the spin. acts a lot like a "Drifter" on a monohull, it's the same thing with a different name. Cats don't use a symetrical spin with a pole on the mast like mono's do, our spins. are more like a huge jib, you can even go upwind in light air with a flat one, just like a Drifter on a mono.

The early spins on the H21's were very fat, cut for going deep downwind. The newer spins on the closed course racing cats (Inter 20's, F18's, F16's, etc.) are much flatter cut, but they also snuff into a sock on the pole, but the older H21 spins went into a bag on the tramp. That's 'crew work'!

If you are going to singlehand it most of the time, you'll want to get a sock on the pole, it's much easier to get it down and snuffed that way, when you are alone. OR...get one of Rick White's "Hooters", which is basically a roller furling spinnaker/large jib setup.

Either way, you'll have to get a spinnaker custom made, as the old H21's are not being produced anymore, neither are the sails. The spinnakers were add-on's for distance racing back in the day. Randy Smyth made a bunch of them, see if he's got any laying around you might be able to get one cheap.