One out of two Schyte Kama Flex PWM that I own developed a whine. Suggestions for a decent replacement are appreciated.

The Scythe SlipStream SY1225SL12LM-P is probably the cheapest replacement from sources such as this. However the MTBF for the SlipStream is quoted as 30,000 hours and in view of your experience with the Kama Flex you may prefer to spend more money this time. If the 300/1300 rpm range is OK I would suggest the Noctua NF-P12-PWM or if you would prefer 300/1500 then Noctua NF-F12-PWM. Agreed these prices are somewhat higher than the Scythe but both Noctua fans includes a comprehensive fitting kit including soft mountings, a PWM Y cable, PWM extension cable and a PWM speed limiting cable. The MTBF is listed as more than 150,000 hours and there is a 6 year warranty.

My requirements are no larger than 750 rpm.

If you run the Noctua NF-P12 PWM under PWM control using the supplied speed limiting cable its maximum speed would be restricted to around 900 rpm.