OK, the guy riding and writing the 'Africa with a surfboard' blog is also posting on Advrider. I asked him about buying a bike and engine size and this is what he had to say:

A 125 is OK, but on the big roads with big trucks, Mike was pretty glad to be on the 250. No idea what the process is in Kenya, but in SA, the only way we got the registration done quickly was to have a guy who was a resident let us use his address.

Obviously, 50cc is out, but 125cc should be taken off your short list, too, based on the above.

Why not just take the airplane and get it over with? Ever heard the concept of "the journey is the goal, not the final destination"?

I have been traveling around europe and india on 125 and 150cc bikes, there have been people doing RTW trips on small machines with great success and having tons of fun!

From time to time i have considered getting a 50cc machine, nor for the sake of being eccentric but to spend more time on the road, seeing more details, making more frequent stops etc. If my knees weren't damaged i would have traveled on a bicycle.

oh please, don't take literally every metaphorical slogan you find on the internet. If you travel without destination you're just a drifter or a bum.

Bum?
 

Well, each to his own ... to me, drifting is fun. If this makes me a bum, I don´t give a rat´s ass.

Thank you guys for all the informations and opinions and especially yuma for your understanding... I already found a way to get a bike in kenya. but i will look into s.a. as well and will contact the surfboard guy soon... thanks

Is it a new bike or used? What kind of costs? What size engine? I would think that SA would be straightforward as long as you can find a local citizen whose address you can use, which is what the surfer guy said his friend did. Like I mentioned earlier, the northern SA city of Johannesburg is in relative close proximity to Botswana; the only thing is you would have to reverse your trip which might or might not fit your plans.

while you might find some countries that let you ride a bike that exceeds your licence don't expect to be covered by your travel insurance if you injure yourself.