Actually, most of them fly in 172's, 182's and sometimes King Air's and helicopters. I have never personally done this, but I flew with a captain who used to be a missionary pilot, so I will tell you what I know. Missionary pilots are usually employed to move people and cargo in support of missionary's in remote areas where moving them by road would be less practical. They fly in remote areas of Africa and Asia and other countries with food, personell, supplies, medicine, and sometimes the natives if they need specialized care. Missionarys help locals get supplies and assistance, as well as learn their language so they can translate the Bible into their local language.
As my friend told me, it's a tough job, as you have to be a certified aircraft mechanic. You are often alone, flying with two radios on. One is for ATC, and one is for position reports to your support team. It's actually very busy work, since most areas have no radar coverage. As you can guess, they have little in the way of weather reporting and ways of finding you if you go down as well.
I'm told it's a very tough job, and not for the faint of heart. You have to live in the bush, and you might not see many familiar things for a long period of time.
I would suggest you look at maf.org for any other questions, and you can email them if the website doesn't help. Good luck, and I hope this helps you.