Is the diameter of the tube itself 90mm, as well? If so, then the focuser used on the Celestron/Skywatcher 100mm ("4-inch") refractors will fit right in place, perfectly, with no adapter needed. Not sure about the focusers used on the 120mm Syntas -- they may larger (to fit 100mm tubes and flanges). The focusers on the 150mm Syntas are *definitely* larger.
JMI, Burgess, William, Intes and Feathertouch focusers -- all of these can be adapted to fit your 90mm Meade.