Keep the chain loose very loose , as it is supposed to be able to touch the swing arm. Too tight and it wears the output shaft badly.
Fit a mizuki vacuum fuel pump (which has no moving parts) , Africa twins (the RD07 mostly) had a fuel pump issue which caused them to weld themselves together and fail , or bring a spare. These things die every 20,000 miles lest you replace it.
I would also change the fuel filter to a different fatter one and reposition it to be far away as possible from the rear cylinder as it gets hot and you can get vapour lock.
Change the reg/rec or take a spare the RD04 has the reg/rec just above the exhaust , it gets shaken apart by the vibrations and also the heat off the exhaust. It is best to re-wire this to the left side instead and put some cush rubbers on it to protect it from vibration.
Take the starter relay off (its the first thing off the +VE on the battery) and shake it if you can hear small bits inside change it or bring a spare.
Bring a bead breaker for the rear tyre as they cannot be broken on the road with your hands and feet.
If you are crossing central America I would probably fit a second fan to the radiators as I recall my africa only had one fan.
Bring along some different jets , in that if you cross the spine of the Andes you will need different jets to deal with the high altitude.
Wrap around hand guards , if you drop the bike the std hand protectors will not protect the levers.
Cable repair kit, as the clutch springs and the throttle springs are quite meaty and wear the cables fairly quickly.
Thats it really the Africa is a VERY tough bike