Mogas is probably just as problematical as Avgas, and most vehicles in the bush run on diesel (if they run at all).
Even in Port Moresby Avgas is difficult to obtain; elsewhere it is just impossible.
Out of curiosity what would you plan to do on this trip? Any notion that it is an idyllic paradise should be dismissed immediately. Whilst PNG has some stunning scenery and some decent people, the risks are very high and the frustrations are immense.