Hi Green
Motorcycle riding in Kenya is a whole new world of experience. It really depends on what you want to do exactly - whizz around Mombasa, or go on a major safari?
If you're going to use it for local transport, I would suggest that you do not for two reasons. Firstly, it is likely to be a very expensive way of doing it, it will be cheaper to get a taxi all the time. Secondly, you're very unlikely to survive more than a week. The main difficulty you'll face either way is actually getting hold of a decent bike to rent withouth buying one. If you're planning to do some major safaris by bike, there are a couple of very important factors to bear in mind. Firstly, no game reserve or national park allows motorcyclists. Secondly, you will need to have very good experience at off road riding, especially on sand, in mud, and on rocky cross country terrain. Without this experience you are very likely to kill yourself. Thirdly, you will need to know your way around a little - you're best off going with someone local. There are no sign posts anywhere in Kenya, few detailed maps, and as soon as you leave a major thoroughfare, you either need to speak the language, know your way around, or carry (and be very familiar with) a GPS.
You will also need to carry serious amounts of fuel, and more importantly, water. You will also need to carry at least 4 inner tubes, two tyres, an airpump, a puncture repair kit and be familiar with motorcycle maintenance. ie its a major mission. If you're serious about doing this, then there are two operators of motorbike safaris in Kenya - one is a French guy:
http://www.motorbike-safari.com .
and the other is an American based in Arusha, Tanzania - he advertises online somewhere, I'll try track him down, his name is Buddy. Without wishing to knock them too much, only the American can really be classified as a true 'local' but frankly, he is not very experienced in the bush and has not been running his business for very long. There is one other guy in Arusha who does not offer commerical tours, but has serious contacts with most private ranchers in Kenya and Tanzania, and can get you access to some great game viewing and some spectacular motorcylcing routes - he has a lot of experience in this, but does it for fun, not for commericial gain.
If you are really serious, then send me a private message and we can see what can be organised. I might as well tell you straight that none of these options is likely to be shoestring cheap. One other thing you could look into is head to Nairobi and on Sundays, they have Motorcross racing, usually out in Karen. The Association of Kenya Motor Sports should be able to tell you where and when. Everyone who has anything to do with bush motorcycling goes to these events, its quite a tight knit community. You will almost certainly be able to hook up with some guys who do some motorcycling at weekends around the Rift Valley and other places like that.