You have a G/S right?
You can get good 18,19 and 21 (not 17) (Michelin Dessert, Pirelli MT21++ but expensive) from a guy called Ian Duncan in Nairobi, he runs a KTM-shop. Ask for Chris at the BMW-shop and he will tell you how to get there.
Ian also have some good Bridgestone mx-tubes which I find better then Michelin tubes.
I guess Chris still have some parts for your bike, like oil-filter, air-filter and some gaskets but don't expect to much :-)
The stretch from Isiolo to Moyale eats tires and it's not the best place to get a puncture. Ethiopia is pretty bad for the tires to so it's a good idea to buy something good. I started from Nairobi on new MT21 and they lasted back to Norway and another 3000 km but then they were slicks :-)
Uganda is great fun and it's mostly eart-based roads so it's not to bad for the tires. You should not miss Queen Elisabeth NP, it's the only park in Africa (I think) where you are allowed to drive the bike and watch lions, elephanths and cheetah. Murchinsons falls are great, hmmm and Jinja... Uganda is a highlight..