PNG- Down the Sepik river and do i have to have a return ticket?

Dear all. Soon am to go to PNG. My first thing is: according to the PNG authorities you must have a return flight ticket in order to enter the country, as far as the a people who have been there experiance goes.. is this enforced?? Id rather buy the return ticket towards the end of my trip.
I am thinking to go down the sepik river by my self either by maybe buying a bigger kayak or maybe maybe a motor engine boat? Any one ever done that? How will the prices go?. In any way i am thinking of carrying a few liters of fuel to the region from some where . What would be a good place? What is the fuel consumption of the motor boats people write on taking.?
Besides all the i am going solo and will try to climb and walk some mountains and treks. And the only big gear question i have is should i bring my tent with me?

Thanks

Yoav

On cross-country treks you can stay in villages, but for climbing mountains I would bring that tent.
Be prepared to have to hire a guide/porter for hiking, both for route-finding and security.

The only tourists I have ever met who actually tried paddling down the Sepik (from Green River) had their canoe turn over on the 2nd day and had to be rescued by passing locals, finishing their journey riding with those folks to Ambunti. If opting for a motorized one, it is probably a better idea to hire it along with its local owner who knows the river. Many interesting villages are not visible from the main river and easy to miss if you don't know where they are.

As for the return ticket, not only can PNG immigration demand it, but so will almost certainly the check-in staff at the AU airport you will probably fly to PNG from.

Thanks Laszlo... It is very helpful... What do you think about purchesing the fuel in advance like in Wewak? Do you think its posible the fly river thw same way???

Yoav

People in the river villages do have fuel.
The Fly River should be if anything easier, but probably less interesting (little if any art).