Should not be a problem. I have used both the Bangkok Airways fly to Trat option combined with a speedboat to Ko Chang and a car and driver. Car and Driver is more or less the same amount of time (perhaps 45 minutes more) when factoring in the arrive at airport/check in/security/boarding/queue at BKK to take off/arrival at Trat/get luggage/get driver to pier/get speedboat/arrive at Ko Chang.

I like the car option as it is a bit more relaxing. Costs total around 3800 THB (or less depending on the car you want) including ferry.