I don't think there are any 145 operators in East Africa - all the administrations there oblige operators to go onto the local registry, so there are not enough EASA aircraft to make 145 registration worthwhile.
There are a few in West Africa and South Africa. Try here:
http://www.easa.europa.eu/approvals-...A_Part_145.pdf
For the Diamond, it might be worthwhile contacting Urbe-Aero at Wiener Neustadt, as they can get authorisation to carry out maintenance on their 145 overseas in urgent cases - but it will be expensive.
Surely best to just carry out a 100 hour service before you go? You would have 120 hours to play with which should be enough to get to Joburg.