What type of preamp do I use for an Electret stylus ?

Based on my use of electret condenser microphones, I’d say a special preamp is not necessary. Any phono preamp should work.

So a MM or MC preamp will do ? 
Does the electret stylus still produce Riaa equalization? 

It’s the same as a MM !
https://www.adelcom.net/MicroAcousticsCart1.htm

Perfect ! Thanks. 

Based on our experience with NOS electret condensor microphones from the early 1970s (Sony ECM-22Ps) used from 1990 to current date, the electret polarizing voltage slowly degrades over time, so you may find that the output level varies from channel to channel, and/or the low bass response is impared particularly in one channel (as the loss of capacitance is probably not uniform).