I have to say that I have no personal experience of fishing small, crocodile-infested creeks in a low-sided tinny. I have heaps of experience fishing freshwater creeks and rivers in a 3.45m tinny with a 15HP Yamaha, tiller operated, outboard. dirvin21 has a tinny that I would be looking at feeling safe in. Ask him for details
Don't neglect to factor in the higher than average tides and associated stronger currents that can exist in tributaries up North. I have a vision of a strong current, a very small motor and a vessel that could easily be turned over. No need to paint the picture any more than that really. A lightweight tinny with high sides is a safer bet, particularly if there are 2 or more occupants...accompanied by at least a 20HP outboard. Maybe 4.2m -4.5m with high sides.
I don't wish to put a damper on your trip, just to encourage you to anticipate the possibilities. It's too late if something does happen.
Good luck, have fun, and keep safe.
bn