I know there were a couple in Kenya with RDA-6 engines.
There were indeed two with RDA-6 engines operating in Kenya a few years back. One (5Y-BRN) operated briefly for East African Safari Express on its short-lived Nairobi-Rumbek service around 2005/6. One of the two (not sure which) is still sitting at Wilson minus props. The 540 ones are powered by RDA-7's.