We arrive in Rome Fiumicino Airport at 10:20 AM, 10/30/22. We need to get to Tiburtina Train Station for a train trip to Venice. Tiburtina train leaves at 1:05 PM.

have I allowed myself enough time to get from the airport to the train station? What might be the fastest mode of transportation?

Any particular reason you are traveling out of Roma Tiburtina as opposed to the main train station, Termini?
Personally I find boarding a train at a terminal station where the train is there waiting for a while much simpler than jumping on a train that is only in the through station (Tiburtina) for a few minutes at most.

There are express trains from FCO to Termini every 15minutes, and it takes 32min total.

To answer your question though, fastest way would be to take a direct train from FCO to Tiburtina. There are faster direct trains that take 32min, and slower ones that take 48. The last one you could take and catch your 13:05 connection departs FCO at 11:53.

I think Termini would give you more options with the frequency of the Leonardo Express from FCO

As for whether you have enough time, that's a wildcard. To make that 11:53 connection you'd need a lot of things to go your way time wise... On time arrival, no wait at immigration, no wait at baggage claim. On recent trips, I've had my bags on the carousel within 30min one time, and another it took over 90. ymmv.

The more important question to ask is "what will I do if I miss that train?"

Tiburtina is fine. It's less hectic than Termini. It's smaller so it'll be easier to find your platform. Rome is not the terminus for that train. Naples is.

To get to Tiburtina at the airport take the FL1 train not the express. It's a fifty minute ride.

Do you have enough time? If your flight arrives on time and you clear immigration quickly. Should be plenty.